A deoxyribonuclease was partially purified from the free-living nematode Caenorhabditis elegans. The DNase functioned as an endonuclease and introduced both single-strand nicks and double-strand breaks into DNA. The enzyme hydrolyzed double-stranded DNA seven times more rapidly than single-stranded DNA. DNase activity was not affected by the addition of divalent cations below 1 mM but was inhibited at higher ionic concentrations. In addition, the enzyme was not inhibited in the presence of 10 mM EDTA. The enzyme was inhibited by salt concentrations greater than 20 mM. Three independent mutations in the nuc-1 gene were shown to reduce nuclease activity to less than 1% of that seen in wild-type organisms.