Competitive inhibition of acid production by thiobacilli was investigated by encouraging the growth of potential competitors. Two-organism competition experiments (Thiobacillus sp. vs. heterotrophic yeast) were conducted in a bench-scale, continuous-flow reactor. Results were sensitive to the influent ammonia/glucose ratio. Under NH4+-limiting conditions, the concentration of T. thiooxidans or T. neapolitanus and acid production rates were reduced by about two orders of magnitude. Under carbon-limiting conditions, only modest reductions in the thiobacilli cell density and effluent sulfate were observed. In general, encouragement of microbial competition can interrupt acid production by thiobacilli and may inhibit crown corrosion in concrete sewers.