Abstract: The purpose of this work is intended to determine the acute and subacute toxicity of ammonium chloride on grass carp (Ctenopharyngodon iddellus). The acute toxicity tests were carried out by using 26-, 47-, 60- and 125-day-old fingerlings, while the sublethal tests were done on 60-day-old fingerlings for 45 days. Fingerlings were exposed to unionized ammonia with concentrations of 0.970, 0.455, 0.099, 0.054 and 0.012 (control) mg NH3/l. The 96-hour LC50 for 26-, 47-, 125-day-old fingerlings were 0.570, 1.609, 1.683 mg NH3/l and the 48-hour LC50 for 47-, 60-, 125-day-old fish were 1.727, 2.050, 2.141 mg NH3/l, respectively. Thus, the results of acute toxicity tests show that the 26-day-old juveniles are more sensitive to ammonia than the older ones. With regard to the subacute toxicity test, the maximum allowable toxicant concentration (MATC) of un-ionized ammonia for grass carp fingerlings was 0.054—0.099 mg NH3/l, as judged from the toxic effects of ammonia on their growth and the gill structure.