Three types of microtubules are seen in the neuronal processes of the nematode Caenorhabditis elegans. Single cytoplasmic microtubules of most neurones have 11 protofilaments whereas those of six touch receptor cells have 15 protofilaments. The axonemes of sensory cilia have nine outer doublets with a variable number (up to seven) of singlet microtubules. Mutations in 11 genes affect the appearance of these microtubules.