Cyathostomins are the most prevalent parasites in horses causing colic, diarrhea and death. The control is difficult due to the large number of different species and routine treatment has favor the selection of resistant population, especially in Brazil [1]. One of the problems is the late diagnostic to monitor resistance and molecular tools must be developed to detect specific alleles (SNPs). Point mutations on the b-tubulin isotype 1 genemay inducemutations on the 167, 198, and 200 codons, which are associated with benzimidazole (BZM) resistance. The TTC/TAC polymorphism at the 167 position has being incriminated as the mechanism of resistance to the BZMs [2]. The objective of this work was to determine the frequency of the polymorphism at the codons 167 and 200 of the b-tubulin isotype 1 gene in BZM-resistant adults and larvae of Cyathostomin sp. Twenty adult worms were collected from one horse from the Sao Jose da Serra Stud, a BZM-resistant (0% to fenbendazole) farm. The larvae were collected from 10 pool samples from BZM-resistant horse farms after faecal egg count reduction test (FECRT). Specific primers [3] were used to amplify the codons 167 and 200 for b-tubulin isotype 1 gene using, 167: forward primers 5’-GCTAACT CACTCACTTGGAGGA-3’; reverse: 5’-CTTTGGTGAGGGAACA ACG-3’ and 200: forward primers 5’TACAATGCTACCCTAT CCGTTCAT-3’; 5’-GAAGTGAAGACGAGGGAATGGA-3’. Samples were sequenced with ABI3130 and analyzed with Mega 5. It was possible to identify the DNA amplified bands (167 and 200 1⁄4 TTC) to all samples (Figure 1) with double peaks 167 (6/20) and 200 (2/20) which corresponded to heterozygous worms (TTC/TAC) on chromatograms. All larvae had double peaks for the 167 and 7 for the 200. We found only the homozygous (TTC/TTC) susceptible genotype in adults and also the heterozygous (TTC/TAC) to benzimidazole in larvae. As suggested by other authors we consider that themutation at the codon 167 may be involved with the mechanism of resistance in the Cyathostomin sp. and that molecular markers using SNPs could be used to detect BZMresistant worms, most certainly for the codon 167 in adults and larvae.