In this study, the cause of the deaths in broodstocks and larvae in rainbow trout hatchery in Van province were investigated. Necropsy was done in broodstocks and larvae. The causative agents and symptoms were evaluated. Dissolved O2, CO2 and NH3, NO2 and NO3 concentrations in water were determined in order to evaluate the environmental factors of the disease. In addition, water temperature, pH and total hardness were also determined from other quality water criterias. At skin and gill, eye edges, fins of broodstock were seen embolus. Around the yolk sac of the larvae were determined the gas bubbles. It was also observed that larvae could not consume the yolk sac, and larval posture was reverse at water surface. The dissolved O2 saturation at the inlet of the pools were measured as 165±2,51% and at the out of broodstock pool was 147±4,16%. The morbidity of the disease in the broodstocks were 25%, the mortality rate was 10%, and the morbidity and mortality rate was 5% in the larvae. According to the findings the disease is not an infectious disease but a non-contagious, environmental gas bubble disease. As a result, the supersaturation of dissolved oxygen in water caused the gas bubble disease in broodstocks and larvae. For the solution of gas bubble disease, it may be recommended to the use of saturation column at the water inlets or aeration of water at the inlets of pools.