Abstract Phytium spp and isolates within species differed in susceptibility to the mycoparasite Pythium oligandrum Drechs., as evidenced by their degree of inhibition by it on cellulose and ability to support its growth across their colonies on agar. Yet no Phythium sp. was highly susceptible to it, and P. graminicola Subramanian was highly resistant. No evidence was found that P. oligandrum produces toxins active against other fungi. In liquid culture, P. oligandrum grew on undisturbed colonies of Phialophora sp. (highly susceptible) but not P. ultimum Trow or Fusarium culmorum (W. G. Sm.) Sacc. (moderately resistant) and not Rhizoctonia solani Kuhn (highly resistant). It grew in culture filtrates of Phialophora sp., P. ultimum and F. culmorum, utilizing organic nitrogen and thiamine released by these fungi, but not in culture filtrates of R. solani. It grew on mycelial macerates of all these fungi, though poorly on those of R. solani. Resistance to parasitism by P. oligandrum seems to reside, at least partly, in low levels of nutrient release from host hyphae.