A depressed growth rate in the absence of pathological changes was observed in the larvae of a marine teleost, red sea bream Pagrus major, exposed to 0.002 and 0.02mg/l NH₃-N (non-ionized ammonia) for 72 hours. Such levels of ammonia are frequently encountered in practical larval culture facilities in Japan and may have a detrimental effect on production. In a second experiment a histological study revealed that three-day-old larvae exposed to 0.05, 0.08, and 0.15mg/l NH₃-N for 24 hours developed cytoplasmic vacuoles in the chondrocytes. The possible causes of such pathology and its relationship to the observed depression in the growth rate are discussed.