In small streams of Donbass on pupae of black flies Odagmia ornata Mg. and Chelocnetha angustitarse Lundstr. occur the larvae of water mites Sperchon setiger S. Thor. and Sperchonopsis sp. n. which pass from pupae to imago during the emergence. The infection extensiveness of adult black flies amounted on the average to 18--20% and intensiveness-- to 1--7 larvae. Larvae of water mites were not recorded from pupae and imago of Simulium argyreatum Mg. and of the genus Wilhelmia. Under laboratory conditions parasitism of water mites larvae on hungry adult black flies caused their quicker death (8--22 hours) as compared to noninfected individuals (1 to 2 days).