The initial hypothesis that predation pressure should decrease with decreasing pH in aquatic macrobenthic communities if predatory invertebrates are more sensitive to water acidification than prey invertebrates is tested. Short-term toxicity bioassays were conducted in soft water (average value of total hardness 38.0 mg CaCO3/L) to determine the differential sensitivity of the predator, Dugesia dorotocephala (Turbellaria, Tricladida), and the prey, larvae of Cheumatopsyche pettiti (Insecta, Trichoptera), to low pH. Test pH solutions were prepared with sulfuric acid (H2SO4). Test species were also exposed to high concentrations of sulfate ion (95 mg SO4 =/L for D. dorotocephala and 340 mg SO4 =/L for C. pettiti) as sulfate toxicity controls, using potassium sulfate (K2SO4). No mortality was observed during these toxicity controls, indicating that toxic effects generated by low pH were fundamentally due to H+ ions. The 72 and 96-h LC50s (as pH values) and their 95% confidence limits were 4.88 (4.72-5.05) and 5.04 (4.89-5.21) for D. dorotocephala, and 3.25 (3.00-3.51) and 3.48 (3.24-3.73) for C. pettiti. Net-spinning caddis-fly larvae migrated from their retreat nets and protruded their anal papillae before dying. After short-term bioassays, predation-pressure laboratory experiments were performed for 6 days. The cumulative mortality of C. pettiti by predation of D. dorotocephala decreased with decreasing sublethal pH values. The average predation rates at mean pH values of 7.7, 7.7, 6.6, 6.5, 6.2 and 6.0 were 2.5, 2.0, 1.33, 1.17, 0.67 and 0.33 larvae/day, respectively. The major biotic factor affecting predation pressure appears to be the reduction in the physiological activity of triclads at low pH.(ABSTRACT TRUNCATED AT 250 WORDS)