The acute toxicity of un‐ionised ammonia to seven New Zealand indigenous fish and one indigenous crustacean species was established. Shrimp (Paratya curvirostris) was the most sensitive species, followed by banded kokopu (Galaxias fasciatus), common bully (Gobiomorphus cotidianus), common smelt (Retropinna retropinna), redfin bully (G. huttoni), inanga (Galaxias maculatus), and longfin and shortfin eels (Anguilla dieffenbachiiandA. australis). The 96 h LC50at 15°C and pH 7.5 and pH 8.1 ranged from 0.75 to 2.35 mg NH3I−1for these species. Unusually, New Zealand fish were more tolerant to ammonia than some native invertebrate species tested by a similar method. Within the limits of the life stages and species tested here, adequate protection is provided for native fish by adopting US Environmental Protection Agency ammonia criteria.