The chronic toxicity of total ammonium and unionized ammonia (NH3) to the native New Zealand freshwater fingernail clam Sphaerium novaezelandiae was assessed in soft water under laboratory conditions. Control survival after 60 days was high (93%) and concentration-response relationships showed the sensitivity of S. novaezelandiae survival was markedly greater to both total and unionized ammonia (6.4x and 4.6x) after 60 days compared with the 30-day exposure at 20 degrees C. Chronic mortality and number moribund (inability to rebury) showed similar sensitivities, but reproduction was a more sensitive endpoint based on a concentration-response analysis. The survival LC50 values for total and unionized ammonia were 3.8 mg (N)/L (pH 7.5) and 0.037 mg (NH3-N)/L, and reproductive values 0.80 mg (N)/L (pH 7.5) and 0.013 mg (NH3-N)/L at 60 days. No observed effect concentration (NOEC) values for both survival and reproduction were 0.97 mg (N)/L and 0. 011 mg (NH3-N)/L, and the lowest observed effect concentration (LOEC) values were 5.4 mg (N)/L and 0.046 mg (NH3-N)/L for survival after 60 days, giving a calculated threshold effect concentration (TEC) of 2.3 mg (N)/L and 0.022 mg (NH3-N)/L. Comparison of the S. novaezelandiae chronic ammonia sensitivity data with the US EPA criteria showed the survival and reproduction TEC values for total ammonia were 1.9x higher than the chronic criterion, and the lethality value 1.4x above the unionized ammonia criterion. The findings suggest that use of the US EPA criteria would provide minimal protection for S. novaezelandiae for chronic ammonia exposure, and that development of site-specific criteria, covering a wide range of environmental conditions, may be required to adequately protect all life stages of this species.