A cytogenetic analysis of the species status of members of the Anopheles dirus group, from natural populations, over a six-year period has produced biogeographic and behavioural data which are presented herein. Species A, B, C and D have quite distinct geographic distributions in Thailand. Species A is the only species in most of the mainland but is absent from the southern half of the peninsula. Species B is dominant in the far south of the peninsula giving way to species C on the north-east side and rarely occurring on the west side of the peninsula; it is unknown from the northern half or the rest of the country. Species C is known from the middle eastern side of the peninsula and from a site far to the north along the Burmese border. Species D occurs down the mountains along the Burmese/Thai border and along the north half of the western peninsula. Each species appears to have distinct times of biting during the night. They also appear to have differential seasonal abundance. These data together with the differing geographic distributions suggest some implications for Plasmodium transmission and that care should be taken to identify these genetic species during malariometric studies.