1. The toxicity of ammonia to Macrobrachium larvae was tested at pH 6.83, 7.60, and 8.34, and the respective 144 hr LC values were 80, 44, and 14 mg ammonia/liter. 2. Toxicity of ammonia was not due solely to the NH molecule. In solutions of different pH and equal NH concentrations, survival was greatly reduced as NH levels increased. 3. A model is proposed to explain the differential effect of ammonia as pH varies. At higher pH (8.4) toxicity results from copious diffusion of NH into larvae. At lower pH (6.8) toxicity is thought to result from competitive inhibition of Na transport by NH. 4. Retardation of growth was documented in sublethal concentrations of ammonia at 6.8 and 7.6. The average dry weight was about 26% less than that of controls (P < 0.05) after a seven day exposure. 5. Results are discussed relevant to the culture and maintenance of crustaceans, and it is concluded that ammonia will not pose a substantial threat in adequately managed systems.