This is a report of two malaria parasite surveys made in the Amazon Valley during 1942 and 1943. The survey in December 1942 on 19,629 persons was made at the end of the dry season when the prevalence of malaria was expected to be lowest, and before control or prophylactic measures were undertaken. The survey in June 1943 on 27,103 persons from 37 localities was made at the end of the wet season when malaria incidence might be expected to be greatest. A study of hospital records on malaria cases in Belem before 1942 supported this assumption as to seasonal prevalence. The incidence of 3.3 per cent in June after control measures were instigated was shown to be lower than the incidence of 5.0 per cent at the expected low season in December.