A quantitative relationship was established between the fungicide dosage required to control a soil-borne disease and the concentration of mycelial inoculum in soil. The incidence of pre-emergence damping-off was observed in pea seedlings grown in non-sterile soil artifically infested with a series of concentrations of inoculum of Pythium irregulare or Rhizoctonia solani and treated with a dosage series of thiram or methylmercuric dicyandiamide. Parallel linear dosage–control curves were obtained which indicated that the logarithm of the fungicide dosage at the 50% control level increases proportionately with the logarithm of the inoculum density. This linear relationship provides a basis for evaluation of the activity of fungicides against various pathogenic fungi in soil.