In this paper we describe the use of a free duplication, sDp2 (I;f), for the recovery, maintenance, and analysis of mutations defining essential genes in the left third of Linkage Group I of Caenorhabditis elegans. The lethals were induced in a strain of genotype (sDp2) + /dpy-5 + unc-13/ dpy-5 unc-15 +, using either 12 mM ethylmethane sulphonate or 1500 r of gamma radiation. Lethal mutations linked to the dpy-5 unc-13 chromosome were recognized by the absence of Dpy-5 Unc-13 individuals amongst the self progeny and were maintained by isolating Unc-13 hermaphrodites. These strains – which have two mutant alleles of the essential gene and a wild-type allele on the duplication – are balanced, since crossing-over does not occur between sDp2 and the normal homologues. Using this sytem we have recovered 58 EMS-induced mutations. These have been characterized with regard to map position and complementation. Twenty-nine of the EMS-induced mutations lie to the left of dpy-5 and define 20 complementation groups; 3 were inseparable from dpy-5 and define 3 complementation groups; 21 were to the right and define 17 complementation groups. Among a set of 29 gamma radiation-induced lethal mutations, 17 appear to be single gene mutations or are very small deletions. We estimate that we have identified from one-sixth to one-half of the essential genes in the sDp2 region.