The infection with members of the Tri- chostrongylid nematodes has been frequently reported from sheep and goats. Because of the widespread use of Benz- imidazoles (BZs), the resistance suspected to occur in some worms populations. In this study, we focused on the prevalent nematode, Marshallagia marshalli, from the abomasa of sheep. Samples were obtained from at least 10 infected farms and diagnosed with morphological and molecular methods. For resistance analysis, genomic DNA from pooled adult samples of all farms were analysed for the beta tubulin gene to detect any polymorphisms at codon positions of F167Y, E198A and F200Y. According to the results, seven farms (70%) revealed resistance (R) allele at F200Y with relatively high frequency. No other mutations were identified at the other two positions. Also, except for one homozygous (RR) occasion, the isolates with R allele had heterozygous (RS) genotype. This finding indicates that the worm populations are still affected by drugs of the BZ class. However, the genetic data also notes on devel- oping resistance mechanisms in M. marshalli populations in sheep.

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