Transient receptor potential (TRP) channels mediate light-induced Ca(2+) entry and the electrical response in Drosophila photoreceptors. The role of TRP channels in other invertebrate photoreceptors is unknown, particularly those, exemplified by Limulus ventral eye photoreceptors, in which calcium release from intracellular stores is prominent. We have amplified cDNA encoding three variants of a Limulus TRP channel. LptrpA and LptrpBencode proteins of 896 and 923 amino acids, differing by a 27 amino acid insert within the C-terminus. LptrpC encodes an alternative 63 amino acid sequence in the pore domain compared with LptrpB. LptrpB and LptrpC are present in ventral eye mRNA, while LptrpA is only present in brain mRNA. In situ hybridization indicates the presence of Lptrp in photoreceptors of the Limulus ventral eye. Some canonical TRP channels can be activated by diacylglycerol analogs. Injection of a diacylglycerol analog, 1-oleoyl-2-acetyl-sn-glycerol (OAG), into Limulus photoreceptors can activate an inward current with electrical characteristics similar to the light-activated current. However, simultaneous elevation of cytosolic calcium concentration appears to be necessary. Illumination attenuates the response to OAG injections and vice versa. These results provide molecular and pharmacological evidence for a TRP channel in Limulus ventral eye that may contribute to the light-sensitive conductance.