Populations of adult mosquitoes occurring in the Osijek area in eastern Croatia were monitored from May 1995 to September 2000 with Centers for Disease Control-style traps baited with dry ice. A total of 106,663 adults was caught, ranging from 0 (several times) to 21,120 in May 1995. The mosquitoes captured represented 18 species. Aedes vexans was the most abundant, followed by Ochlerotatus sticticus. Both of these species have breeding sites in flooded areas in the areas surrounding Osijek. Furthermore, these 2 species comprised 86% of total adult mosquitoes collected. The life cycles of these species and their abundances were positively correlated with water level. The peak occurrence of these species was in late spring to early summer. During 6 years, adult mosquito populations were significantly correlated with the water level of the Drava River.