Thirteen species of anopheline mosquitoes were collected in all-night human-bait indoor and outdoor collections at 5 houses from July 1986 through December 1987 in and near the town of Costa Marques, Rondonia, Brazil. Anopheles darlingi and An. deaneorum accounted for more than 92 and 6%, respectively, of all anophelines collected from human-bait outside houses and 90 and 9%, respectively, inside houses in the town. The diversity of anophelines was greater in human-bait collections in rural areas. However, An. darlingi still accounted for more than 90% of all the anophelines collected from human-bait inside and outside rural houses. Data on nocturnal and seasonal biting rhythms for the more common anophelines are given.