Bloodfeeding activity, host preference, and seasonal abundance of the Anopheles dirus complex in relation to environmental factors were measured during a 2-yr period at Pu Teuy Village, Sai Yok District, Kanchanaburi Province, western Thailand. Anopheles dirus s.l. was found more abundant during the wet season compared with the dry and hot seasons. Bloodfeeding by An. dirus s.l. commenced immediately after sunset with a distinct peak of activity at 2000 h. Anopheles dirus s.l. fed more readily on cattle than on humans, demonstrating a significant but slightly stronger zoophilic behavior (P  =  0.02). The biting activity of An. dirus s.l. in Pu Teuy appears to differ from other localities in Thailand, suggesting the existence of significant biological variability within the species complex depending on the local circumstances.