A lectotype is designated for Agabus setulosus (J. Sahlberg, 1895), a rare rheophilous dytiscid known mainly from northern Fennoscandia. The larval instars are described for the first time, based on material from northern Sweden. The first-instar larva is characterized by the presence of two dorsal spines on each femur and a pair of mediodorsal setae on last abdominal segment; both characters are seemingly unique among Agabus. The third instar differs from all other Palaearctic Agabus species in the complete sclerotization of the ring-like abdominal segment 6. The biology of A. setulosus is described from a Swedish population; semivoltine life cycle with overwintering eggs laid in summer followed by adult overwintering prior to breeding is suggested.