Serotonin [5-hydroxytryptamine (5-HT)] modulates feeding activity, egg-laying, and mating behavior in the free-living nematode, Caenorhabditis elegans. We have cloned a novel receptor cDNA from C. elegans (5-HT2Ce) that has high sequence homology with 5-HT2 receptors from other species. When transiently expressed in COS-7 cells, 5-HT2Ce exhibited 5-HT binding activity and activated Ca2+-mediated signaling in a manner analogous to other 5-HT2 receptors. However, 5-HT2Ce displayed unusual pharmacological properties, which resembled both 5-HT2 and 5-HT1-like receptors but did not correlate well with any of the known 5-HT2 subtypes. Two splice variants of 5-HT2Ce that differ by 48 N-terminal amino acids were identified. The two isoforms were found to have virtually identical binding and signaling properties but differed in their levels of mRNA expression, with the longer variant being four times more abundant than the shorter species in all developmental stages tested. Taken together, the results describe two variants of a novel C. elegans 5-HT receptor, which has some of the properties of the 5-HT2 family but whose pharmacological profile does not conform to any known class of receptor.