Frotho I is one of the legendary Danish kings in Saxo Grammaticus' Gesta Danorum, where he has a substantial biography. He succeeds his father Hadingus to the throne and replenishes the war-drained treasury by slaying a dragon and winning its treasure. He uses the money to finance expeditions into the Baltic, where he wins victories with clever strategems (including one where he crossdresses as one of his own shieldmaidens). After some trouble at home he campaigns successfully in Britain and captures London. He finally dies in a war against the king of Sweden.
| The family tree of legendary Kings of the Danes, according to the Gesta Danorum (Books I to VII)
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Kings of the Danes are in bold and marked with an asterisk (*). Kings of the Swedes are marked with a dagger (†).
| | Humble | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | KEY | | | | | | | | | | |
| Angul | | Dan* | | Grytha | | | | | | | | | | | | | | | | Marriage or coupling | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Parent and child | | | | | | | | | |
| The Angles | | Humble* | | Lother* | | King of the Saxons | | | | | | | | | | | | | | Inheritance by other or unclear means | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | Sigtryg, King of the Swedes† | | | | Skiold* | | Alfhild | | Sumble, King
of the Finns | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | Gróa | | | | Gram*† | | | | Signe | | Henry, King
of the Saxons | | Hakon, King
of the Nitherians | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| Swipdag*† | | [unnamed] | | Guthorm* | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | Hadding* | | | | | | | | | Ragnhild | | | | | | | | | | |
| | | Asmund† | | Gunhild | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Guthorm | | | | | | | | | | | | | | |
| Henry | | Uffe† | | Hunding† | | Thorhild | | | | | | | | | Ulfhild | | | Ubbe | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Scot | | | | | | | | |
| | | | | | | | | | | | Ragnar† | | Swanhwid | | | | | | Handwan | | |
| | | | | | | Thorwald | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | Frode* | | [unnamed] | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | Hothbrodd† | | | | | | | | | | | Halfdan* | | Ro | | Skat | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | Thora | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | Helge* | | Ro* | | | | | | | | | | | | | |
| | | | | | Athils† | | Urse | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | King Gewar | | Rolf*† Krage | | Rute | | Bjarke | | Skulde | | Hiartuar†* | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| Hother†* | | Nanna | | Herlek | | Gerit | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | Rorik* | | | | | Gerwendil | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| King of Britain | | Gerutha | | Horwendil | | Feng | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| [unnamed] | | | | Amleth | | Hermutrude, Queen of Scotland | | Wiglek* | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | Wermund* | | | | | | Frowin, governor of Sleswik | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | Uffe*, aka Olaf the Gentle | | [unnamed] | | Ket | | Wig | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | Dan* | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | Hugleik* | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | Frode* the Vigorous | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | Dan* | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| Kraka | | Ragnar | | Brak | | Fridleif* the Swift | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | Gotar, King of Norway | | | | | | | Hun, King of the Huns | | Westmar | | Koll | | Gotwar | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| Roller | | Erik† | | Gunwar the Fair | | Alfhild | | Frode* | | Hanund | | Grep | | | | 11 other brothers (two named Grep) | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | Hythin, King of Tellemark | | | | | Alf | | Eyfura | | Arngrim | | Grubb | | | | | Hiarn* | | Amund, King of Norway | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | [unnamed] | | Halfdan† | | | | 12 sons, including Anganty | | Ane | | Juritha | | Fridleif* | | Frogertha | | Frok | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | Hanef, King of Saxony | | Swerting, King of Saxony | | | | | | Olaf | | Frode* | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | Siward† | | | | | | Unnamed sons | | [unnamed] | | Ingeld* | | Helga | | Helge | | Asa | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | Frode | | Fridleif | | Ingeld | | Olaf* | | Karl, governor of Gothland | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | Ulfhild | | Frode* | | | | | | | | | | Hather, a chief | | Harald* | | Signe | | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | Erik | | | | | | | | | | | | Thorhild | | Halfdan*† Biargramm | | Harald | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | Asmund | | | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | | | | Kings of Norway | | | | |
Name spellings are derived from Oliver Elton's 1905 translation, The First Nine Books of the Danish History of Saxo Grammaticus, via Wikisource.
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