Male mating behavior of the simultaneous hermaphrodite freshwater snail Lymnaea stagnalis is controlled by a neuronal network that consists of various types of peptidergic neurons, as well as serotonergic cells. In the present article, we describe the isolation and characterization of a cDNA clone that encodes a multipeptide preprohormone expressed in the anterior lobe of the right cerebral ganglion, in a group of neurons that principally innervate the penial complex. The preprohormone is 219 amino acids in length and contains 10 copies of the peptide Ala-Pro-Gly- Trp-Gly. Posttranslational processing of the prohormone may lead to the generation of the amidated neuropeptide Ala-Pro-Gly-Trp-amide (APGWamide), an amidated C-terminal anterior lobe peptide, and four connecting peptide sequences, C1-C4. We show by in situ and filter hybridizations that neurons of the right anterior lobe comprise the major site of expression of the APGWamide gene. Expression of the APGWamide gene is detected in the CNS of both adult animals and noncopulating juveniles. Peptides derived from the APGWamide prohormone are probably involved in the control of a part of the male mating behavior and have both central and peripheral targets.