A sterile fungus, designated as StFCh1-1, and two other fungi, Embellisia chlamydospora and Verticillium chlamydosporium, isolated from cysts of the sugar beet cyst nematode, Heterodera schachtii, were tested as biological control agents against this nematode. The combination of StFCh1-1 with either E. chlamydospora or V. chlamydosporium resulted in significantly greater impact on the nematode population than any fungus alone. More females and cysts were colonised by StFCh1-1 than by either other antagonist. These nematodes stowed the typical fungal sclerotium on the outside of the body wall, and the hyphal imprint on the inside of the cyst. Few females developed on sugar beet roots in the soil of those treatments that included the sterile fungus. No juveniles emerged from cysts colonised by the sterile fungus, while some juveniles hatched from eggs within cysts colonised by either E. chlamydospora or V. chlamydosporium. The density of E. chlamydospora and V. chlamydosporium in soil, as measured by colony forming unit counts, decreased over time. Quantification of the sterile fungus in soil was not possible using this technique due to rampant growth of saprophytic fungi on the isolation medium. The results suggest that, despite the promising nature of the sterile fungus StFCh1-1 in comparison with the two other fungi used, a carefully combined inoculum could still be more effective at controlling H. schachtii. The results are discussed in the context of compatibility of these organisms.