Abstract: In this study, several physiological parameters of inoculated sugar beet plants, with the beet cyst nematode, Heterodera schachtii, were evaluated in the presence of an antagonistic sterile fungus StFCh1-1 in the rhizosphere condition. The sugar beet plant used in this bioassay was a multigerm cultivar, 7233, which is sensitive to the beet cyst nematode and has been adapted to cultivate in temperate and cool regions of the sugar beet production areas of Iran. In this regard a potent bioassay was conducted in the sterile glass tubes (20H3 cm Ø) containing 30 g autoclaved soil and planted with a sterile germinated seed of sugar beet, totally in 24 tubes. The seedling of sugar beet plant at four leaves stages in two treatments (nematode and nematode + fungus) was inoculated with 50 disinfected beet cyst nematodes. As a standard procedure the bioassay composed of four treatments including: fungus, nematode, nematode + fungus and untreated control. Two months after nematodes= inoculation some physiological parameters of plants were measured including: total chlorophyll, potassium and biomass. The two treated plants with nematode and nematode plus fungus showed significant decrease in biomass and chlorophyll contents but treatment with fungus alone showed no significant differences in the biomass and chlorophyll content of plants in comparison with the control. The potassium content of shoots in the invaded sugar beet plants was lowest, but it was highest in the roots. These changes might be indication of adaptive osmoregulation or acclimation responses in plants due to the nematodes as bio-stressors through the increase of metabolites and solutes. Also, these results confirmed that in plants inoculated with nematode plus fungus, a few number of female of nematodes were developed due to the antagonistic effects of sterile fungus StFCh1-1. Meanwhile, the fungus didn=t have any detrimental effect on biomass, chlorophyll content and potassium in leaves of sugar beet cv. 2733 and it is safe to use as a biocontrol agent against Heterodera schachtii.