Toxicity of ammonia to early life stages of green sunfish Lepomis cyanellus was studied. One chronic and four acute exposure studies were conducted. During the chronic study green sunfish were exposed to a series of ammonia concentrations from the blastula stage through 40 days post-hatching. During the acute studies, juvenile green sunfish were exposed at four pH levels, 6.6, 7.2, 7.7 and 8.7, to derive 3-, 6-, 12-, 24-, 48-, 72-, and 96-h LC50 and EC50 concentrations of un-ionized (NH3) and total ammonia (NH3(-N). Pre-hatching embryological stages were found to be the most resistant stages studied.