In the present work, I describe an antiserum that specifically stains all neurons in C. elegans. This probe should facilitate developmental studies in this organism in the same way as anti-horseradish peroxidase has in Drosophila. The antiserum was raised against an 8 amino acid peptide representing part of an insect neuropeptide precursor, but there are no indications that this cross-reactivity reflects evolutionary homology. The antiserum allows the whole nervous system of C. elegans, including all cell bodies and processes, to be brightly stained. The subcellular staining pattern suggests that a cytoskeletal component is recognized. I have also isolated a mutation (e2481) that abolishes staining in all but 7 neurons, the 6 microtubule cells and 1 cell in the tail. Finally, I show that the pattern of staining in the nematode P. redivivus is similar to that seen in animals carrying the e2481 mutation.