Parasitic nematodes are major human and agricultural pests, and benzimidazoles are amongst the most important broad spectrum anthelmintic drug class used for their control. Benzimidazole resistance is now widespread in many species of parasitic nematodes in livestock globally and an emerging concern for the sustainable control of human soil transmitted helminths. β-tubulin is the major benzimidazole target, although other genes may influence resistance. Among the six C. elegans β-tubulin genes, loss of
ben-1 causes resistance without other apparent defects. Here, we explored the genetics of C. elegans β-tubulin genes in relation to the response to the benzimidazole derivative albendazole. The most highly expressed β-tubulin isotypes, encoded by
tbb-1 and
tbb-2, were known to be redundant with each other for viability, and their products are predicted not to bind benzimidazoles. We found that
tbb-2 mutants, and to a lesser extent
tbb-1 mutants, were hypersensitive to albendazole. The double mutant
tbb-2 ben-1 is uncoordinated and short, resembling the wild type exposed to albendazole, but the
tbb-1 ben-1 double mutant did not show the same phenotypes. These results suggest that
tbb-2 is a modifier of ABZ sensitivity. To better understand how BEN-1 mutates to cause benzimidazole resistance, we isolated mutants resistant to albendazole and found that 15 of 16 mutations occurred in the
ben-1 coding region. Mutations ranged from likely nulls to hypomorphs, and several corresponded to residues that cause resistance in other organisms. Null alleles of
ben-1 are albendazole-resistant and BEN-1 shows high sequence identity with tubulins from other organisms, suggesting that many amino acid changes could cause resistance. However, our results suggest that missense mutations conferring resistance are not evenly distributed across all possible conserved sites. Independent of their roles in benzimidazole resistance,
tbb-1 and
tbb-2 may have specialized functions as null mutants of
tbb-1 or
tbb-2 were cold or heat sensitive, respectively.