Oospores ofPythium myriotylumin nonsterile soil extracts became invaded by a fungus that was isolated and identified asHyphochytrium catenoides. Sporangia ofH.catenoidesformed as spherical or ovoid lobes within thePythiumoospore and as radial catenulate swellings on agar media. In water, these sporangia formed short exit tubes through which anteriorly monoflagellated zoospores were released. Oospores ofP.myriotylumandAphanomyces euteichesplaced in suspensions of motile zoospores ofH.catenoidesbecame extensively invaded within 48 h. Oospores ofPythium aphanidermatumandP.ultimumwere also attacked, but these species were less susceptible to invasion than wasP.myriotylum. Sporangia, hyphae, and zoospores of thePythiumspp.,A.euteiches, and several other phycomycetes were not observed to be invaded. Zoospores ofH.catenoideswere formed and released in water adjusted to pH 5.0–7.0 and at temperatures of 10–35 °C, but were produced most abundantly and were most highly motile at pH 6.0–7.0 and at 20–30 °C. The roots of several crop plants did not appear to be parasitized byH.catenoides. Oospores ofP.myriotylum,P.aphanidermatum, andP.ultimumin moist soil artificially infested with these species andH.catenoideswere destructively invaded by the mycoparasite. Its presence in several soils and its ability to reproduce quickly in its parasitic stage suggest thatH.catenoidesmay play a role in the natural decline of susceptible Oomycetes in soil.