Two static (renewal system) toxicity bioassays were performed in duplicate to examine the effect of sodium chloride (NaCl) on the tolerance of early and last instar larvae of Cheumatopsyche pettiti to water acidification. 0.5 L glass aquaria were used. 200 mg of NaCl were added to each aquarium at the initiation of the second bioassay. In the first bioassay (with traces of NaCl), the 72 and 96 h LC₅₀s were respectively pH 3.49 and 3.71 for early instar larvae, and pH 3.12 and 3.35 for last instar larvae. In the second bioassay (with a high concentration of NaCl), the 72 and 96 h LC₅₀s were respectively pH 2.80 and 3.10 for early instar larvae, and pH 2.54 and 2.76 for last instar larvae. Early instar larvae appear to be significantly (P < 0.05) more sensitive to low pH than last instar larvae. Moreover, LC₅₀s (as pH values) in the second toxicity bioassay are significantly (P < 0.05) lower than those in the first toxicity bioassay. It is concluded that NaCl can increase the tolerance of larvae of C. pettiti to water acidification. This ameliorating effect could be due to an improvement in the ionic regulation of larvae.