The tolerance of spiny lobster J. edwardsii phyllosoma to the effects of increasing levels of ammonia was assessed at Stages I to IV of larval development. The median lethal concentration (96-h LC₅₀) for total ammonia (and corresponding NH₃-N) were 31.6 (0.97) mg l⁻¹, 45.7 (1.40) mg l⁻¹, 52.1 (1.59) mg l⁻¹, and 35.5 (1.01) mg l⁻¹ at Stage I, II, III and IV, respectively. When Stage II larvae were cultured through to Stage III at total ammonia concentrations of 0.5 (control), 1.4, 3.8, 6.3 and 9.5 mg l⁻¹, the intermoult period increased significantly at and above 6.3 mg l⁻¹ (from 11.2 days in controls to 13.2 days at 9.5 mg l⁻¹). Thus, the no-observable-effect-concentration (NOEC) of total ammonia (and corresponding NH₃-N) at Stage II of 3.8 mg l⁻¹ (0.12 mg l⁻¹) was used as the divisor in the acute:chronic ratio (i.e., 96-h LC₅₀ ÷ NOEC) to estimate the concentrations at Stages I, III, and IV, which were 2.7 (0.08) mg l⁻¹, 4.4 (0.14) mg l⁻¹, and 3.0 (0.09) mg l⁻¹, respectively. These results provide the basis for the design and management of systems for the culture of larval J. edwardsii.