The complement of beta-tubulin alleles in Trichostrongylus colubriformis populations was examined and found to undergo changes similar to those previously reported for Haemonchus contortus following selection for benzimidazole (BZ) resistance. Genomic DNA from BZ-resistant and -susceptible strains was probed with a series of overlapping fragments derived from a T. colubriformis beta-tubulin gene. A susceptible population showed a high level of polymorphism (detected as RFLPs with several enzymes and directly by sequence analysis) at a locus, tcb-1, which appears to be the homologue of the gru-1 locus in H. contortus. This polymorphism disappeared following selection for BZ resistance, leaving a single tcb-1 allele in the resistant population. The same single allele was present in 2 additional, unrelated resistant populations. These data support the hypotheses that tcb-1 and gru-1 are major determinants of BZ susceptibility and hence a major target of BZ-resistance selection.