PTL-1, a microtubule-associated protein of the structural MAP2/tau family, is the sole member of this gene family in Caenorhabditis elegans. Sequence analysis of available invertebrate genomes revealed a number of single, putative tau-like genes with high similarity to ptl-1. The ptl-1 gene is expressed in a number of cells, most notably mechanosensory neurons. We examined the role of ptl-1 in C. elegans in adult neurons as well as during development. A ptl-1 knockout strain of worms exhibited an egg-hatching defect, as well as a reduced sensitivity to touch stimuli. In addition, the knockout allele ptl-1(ok621) acts as a dominant enhancer of several temperature-sensitive alleles of mec-7 and mec-12, which code the isoforms of beta-tubulin and alpha-tubulin that together form the unusual 15 protofilament microtubules involved in touch sensation. These results demonstrate for the first time a functional role for this microtubule-associated protein in nematodes and suggest that PTL-1 is involved in mechanosensation as well as some aspect of embryogenesis.