The 3rd-instar larvae of 8 Fennoscandian species of Hydroporus, including the type species of the 2 subgenera Sternoporus Falkenström and Hydroporidius Guignot, are described based on material from N. Sweden. Larvae were reared from eggs laid in captivity, or collected in the field and identified after pupation. A key for the identification of Hydroporus larvae is provided, including 20 of the 34 species known from Fennoscandia. Data on phenology and habitat distribution of the species described are given. Three different life-cycle types are recognized in the genus. The subgeneric classification is discussed in the light of larval morphology. The subgenus Hydroporidius Guignot, 1949 is made a junior synonym of Sternoporus Falkenström, 1930. The species of the memnonius-group are transferred from Hydroporus (s. str.) to Sternoporus. The isolated position of H. picicornis is noted.