Abstract This work was conducted to investigate high mortalities of eyed-eggs and fry in trout hatcheries during the winter seasons of 1994-1996. Low water temperature (0-6 °C), clays covering eggs, directly sun-light and mechanically made faults were observed as the reasons of mortality in brook trout (Salmo trutta) eyed-eggs and fry In one of the hatcheries. Reddening and haemorrhages around eyes of eggs, became hard in yolk-sac of fry. Vibrio parahaemolyticus infections were present in rainbow trout eyed-eggs (Oncorhynchus mykiss Walbaum) in the other hatcheries. Infections caused reddening and haemorrhages around eyes of eggs. Bacterial isolates were identified in each hatchery, and tested to determine sensitivity against to 12 chemotherapeutants. Also, pathogenicity of two isolates were examined using healthy rainbow trout fry. In a. therapeutic experiment baths with chloramine-T and norfloxacin controlled the natural V parahaemolyticus infections.