We have sequenced 45 mutations in mec-7, a beta-tubulin gene required for the production of 15-protofilament microtubules in the nematode Caenorhabditis elegans, and have correlated sequence alterations with mutant phenotypes. The expression patterns of most alleles have also been determined by in situ hybridization and immunocytochemistry. Most (12/16) complete loss-of-function alleles, which are recessive, result from nonsense mutations, insertions, or deletions; three others disrupt a putative GTP-binding domain. Three of the four loss-of-function, missense mutations result in elevated mec-7 message levels, suggesting a defect in tubulin autoregulation that may be attributable to a loss in the ability to form heterodimers. Most (8/9) mild alleles are caused by missense mutations. Two mild alleles appear to increase microtubule stability and lead to the elaboration of ectopic neuronal processes in mec-7-expressing cells. Most (15/23) mutations that cause severe dominant or semidominant phenotypes are clustered into three discrete domains; four others occur in putative GTP-binding regions. Many of these dominant mutations appear to completely disrupt microtubule assembly.