Several earlier researchers have pointed out that trout and salmon fry hatched under gravel are hardier than those reared in a hatchery. The present report confirms these observations, and tries to delineate the effect of certain environmental factors on the embryonic development of salmon fry. Salmon fry (Salmo salar) hatched under gravel were better developed, more deeply pigmented, and on the average 15 percent heavier than those hatched in troughs. The average number of vertebrae was significantly influenced by solar radiation and by gravel cover. The separate effects of these two environmental conditions tended to obscure one another. Vertebral malformations were numerous in fry hatched under gravel in total darkness. Mortality, caused by blue-sac disease at the time of the spring run-off of snow water was less among brown trout (Salmo fario) alevins hatched in gravel than among hatchery reared alevins. Rainbow trout (Salmo gairdneri irideus) fingerlings hatched under gravel were less vulnerable to predators (eels) than fingerlings hatched in hatchery troughs.