When formulated as assimilative hyphae in alginate pellets, the nematophagous fungus Hirsutella rhossiliensis was more active (i.e., parasitized more assay nematodes) in an acidic vineyard soil than in a neutral vineyard soil. To determine whether soil pH explained the difference, fungus activity was measured in soil from the neutral site that had been acidified (by adding sulfuric acid) and in soil from the acidic site that had been neutralized (by adding calcium hydroxide). As hypothesized, the activity of pelletized Hirsutella rhossiliensis was negatively correlated with soil pH. Maximum activity occurred at pHcalcium chloride4.5, and activity gradually declined to near zero as the pH increased to 6.5 and rapidly declined to near zero as the pH dropped below 4.0. Assays performed on leached soil samples indicated that the effects of sulfuric acid and calcium hydroxide were largely due to pH rather than to specific ions or osmotic potential. The effect of pH, however, was indirect. Heating the neutral soil to 60°C for 2 h did not alter soil pH or electrical conductivity but increased fungus activity to levels equivalent to those in acidified soil. We conclude that, in these two soils, heat treatment or low soil pH suppresses soil organisms that otherwise interfere with growth of Hirsutella rhossiliensis from alginate pellets.