Dopamine is the putative transmitter of eight neurons in the hermaphrodite form of the nematode Caenorhabditis elegans. These include the cephalic and deirid neurons, which are believed to be mechanosensory. The male has an additional six dopaminergic neurons in the tail. Mutants have been selected which have defects in the formaldehyde induced fluorescence and lack dopamine to varying degrees, but they are not insensitive to touch. The dopaminergic neurons of C. elegans are compared with the homologous neurons in Ascaris lumbricoides.