ABSTRACT Isolates of a sterile fungus designated ARF (Arkansas fungus) can be separated into two groups, ARF-C and ARF-L, that differ morphologically and in their ability to suppress numbers of Heterodera glycines on soybean. Our objectives were to determine if the two ARF groups differed in their ability to parasitize juveniles, females, and eggs in the rhizo-sphere of soybean and to proliferate in soil. The experiments were conducted in a greenhouse using soil infested with homogenized ARF mycelium. The ARF-L isolates parasitized more juveniles and young females than did the ARF-C isolates. Suppression of these stages was 67% for ARF-L and 12% for ARF-C isolates 14 days after nematode inoculation. When soybean plants containing gravid females were transplanted into fungus-infested soil, ARF-L isolates parasitized 55 to 98% of nematode eggs, whereas ARF-C isolates parasitized 0 to 22%. In both heat-treated and nonheated soil, the biomass of mycelial mats, a measure of relative proliferation, tended to be greater for ARF-L than for ARF-C isolates. The ability of ARF-L isolates to parasitize a large percentage of both pre-reproductive stages and eggs of H. glycines may contribute to its effectiveness as a biological control agent.