Gastrointestinal nematodes in sheep represent serious problem in pastoral systems due to various production losses and significant cost in sheep industry worldwide. One of control options for these parasites in grazing sheep is application of nematophagous fungi to reduce number of available infective larvae, where Duddingtonia flagrans is one of the most promising candidates. In order to estimate effect on larval reduction in vitro, coproculture assay was performed with theoretically generated dose of chlamydospores added to faeces. Total reduction rate of larvae in fungus group compared to control was 32.45%, but without statistically significant difference in larval yields at 0.05 confidence level. Observed reduction percentage was lower in our study compared with results of other researchers. Possible reasons for low efficacy of fungus obtained in our study compared to other results are very complex and some factors are discussed.