The natural production of sulfides is responsible for poor channel catfish production in many acid lakes in northeast Texas. The TLmof un-ionized hydrogen sulfide for channel catfish fry ranged from 0.8 ppm at pH of 6.8 to 0.53 at pH 7.8. At pH 7.0 the TLmof this gas is 1.0 ppm for fingerling catfish, 1.3 for advanced fingerlings, and 1.4 for adult channel catfish. Small fish were also killed more quickly when exposed to these concentrations. Maximum concentrations of hydrogen sulfide are produced in the spring. Channel catfish populations can be maintained by continued stocking of adult fish or by raising the pH with agricultural limestone, which in turn lowers the toxic un-ionized hydrogen sulfide.