The mRNA and the bifunctional protein for the two glyoxylate cycle (GC) enzymes, isocitrate lyase and malate synthase, are expressed in a tissue- and stage-specific pattern in Caenorhabditis elegans. Since expression of the two enzymes for the carbon-conserving glyoxylate cycle is regulated by the availability of carbon sources in microorganisms, we have studied the bifunctional GCP gene expression under fasting conditions and in certain mutants of C. elegans in order to understand possible mechanisms regulating its expression during nematode development. The GCP mRNA and protein levels were elevated in early larvae which were never fed, a result consistent with previous enzyme activity measurements (Khan, F.R., & McFadden, B.A. (1982) Exp. Parasitol. 54, 48-54]. However, larvae of later stages also expressed higher levels of GCP mRNA and protein when they were shifted from normal to fasting growing conditions. The GCP expression appeared to be regulated primarily at the transcriptional level throughout development. Although the expression of both the GCP gene and lin-14 peaks at about the same time during development and are induced by fasting, the regulation of the GCP gene is independent of the heterochronic lin-14 control mechanism of postembryonic lineages, as demonstrated by the fact that there was no significant change of the GCP at both mRNA and protein levels in the heterochronic lin-4 (lf) and lin-14 (gf) mutants compared to the wild type.