Using a screening procedure that is based on a histochemical stain for the enzyme β-glucuronidase, we have isolated several mutants of the nematode Caenorhabditis elegans affected in β-glucuronidase activity. All of the mutations fall into one complementation group and identify a new gene, gus-1, which has been mapped on the right arm of linkage group I (LG I), 1.1 map units to the left of unc-54. The mutants have no visible phenotype, and their viabilities and fertilities are unaffected. Linked revertants of two of the mutations have been isolated. They restore enzyme activity to almost wild-type levels; the β-glucuronidase that one of the revertants produces differs from that of the wild type. We propose that gus-1 is the structural locus for β-glucuronidase.