Documents the dominant occurrence of species of the genus Anopheles whose larvae develop mostly in waters connected with the construction or the operation of the individual reservoirs. Highest mean numbers of larvae (35-39 individuals per 100cm2 of grown water surface of drainage channels, maximum up to 141 larvae on a one-time sampling) were ascertained between late June and early August. Occurrence of Aedes spp. decreased due to flooding of their original breeding gounds by the waters of the reservoirs or due to their drying out. Development shows an unequivocal trend towards a bloom of Anopheles and Culex species.