The developmental regulation of Caenorhabditis elegans DNA topoisomerase I expression was examined using synchronized Caenorhabditis elegans cultures. Variations of the relative mRNA and protein levels of the enzyme during their development were measured by Northern and Western analyses, respectively. The mRNA level was the highest at the embryonic stage, decreasing rapidly to the one tenth level at the L1 stage, and then increasing by a few fold at the L4 and young adult stages. The protein level was the highest at the L1 stage, with gradual decreasing at the following stages until it showed a slight increase at the young adult stage. Based on our results of the expressional regulation, the possible roles of DNA topoisomerase I in the development of C. elegans are discussed.