We have found that formaldehyde is capable of inducing mutations in the nematode Caenorhabditis elegans. 4 concentrations of formaldehyde were tested. At a concentration of 1%, formaldehyde is lethal to the nematode, and 0.01% formaldehyde did not induce any mutations in approx. 60 000 tested chromosomes. 2 concentrations of formaldehyde, 0.1% and 0.07%, were found to be mutagenic, inducing both point mutations and deficiencies in the unc-22 region of linkage group IV. 4 of the point mutations have been demonstrated to be alleles of the unc-22 gene and have been mapped within the locus. 2 of the putative deficiencies have been confirmed. Each spans the unc-22 gene and at least 2 other genes in the region. A rough estimate of the forward mutation frequency using 0.1% formaldehyde in this region is 3 X 10(-5), while for 0.07% the frequency is 2 X 10(-4).