Studies on the ecology of Anopheles gambiae s.s. and the transmission of malaria were undertaken in a high rainfall forested area in southern Sierra Leone. Anopheles gambiae s.s., identified by chromosomal techniques as the Forest form, was the only malaria vector in the study village. Surprisingly, rice fields or swamps were not favoured breeding places for this species; breeding mainly occurred in temporary pools. The mean annual sporozoite rate of An. gambiae s.s. determined by ELISA was 3.9%. Pyrethrum spray, human bait, and exit trap collections, as well as identification of mosquito blood-meals using the ELISA method, showed that the forest chromosomal form of An. gambiae s.s. was highly anthropophagic and exophilic.