We have isolated a gene (clone Y113G7A.11) from Caenorhabditis elegans (C. elegans), that we have designated as ceST1, and which is the only member of the cytosolic sulfotransferase (SULT) gene family present in the genome of this organism. We identified the SULT motifs of ceST1 based upon their deduced amino acid sequence, and subsequently expressed the ceST1 cDNA in Escherichia coli and characterized its enzymatic properties. The recombinant protein showed sulfation activity for 4-nitrophenol and 2-naphthol substrates, but did not catalyze the sulfation of either monoamines or hydroxysteroids. Another compound sulfated by ceST1 is bisphenol A, which is known to stimulate germ cell proliferation in C. elegans. SULT activity was not detected in the cytosol of C. elegans, probably due to heat labile inhibitors. The ceST1 protein was detectable in the cytosol of C. elegans using anti-sera raised against recombinant ceST1, and transcripts could also be detected throughout the developmental stages. Moreover, high levels of ceST1 expression were evident at both the embryonic and adult stages and were augmented in dauer larva. These findings suggest that this sulfotransferase either forms part of a defence system against xenobiotics or regulates germ cell proliferation in C. elegans.