Saprophytic colonization of organic material in soil by Pythium mamillatum was shown to depend on the presence of a carbon source within the substrate, but the presence of nitrogen did not seem to affect colonization. Too high a concentration of such a carbon source in the substrate tended to reduce colonization by P. mamillatum , this being ascribed to an antagonistic effect of other soil micro-organisms. Substrates maintained at low pH (3–4) were not colonized by this fungus. High soil moisture content, relatively low temperature, reaction between pH 5·5 and 7·0, and fine soil texture were found optimum for saprophytic colonization by P. mamillatum .