Pheromones play a significant role in coordinating reproductive activity in the marine opisthobranch mollusk Aplysia. Although solitary during most of the year, these simultaneous hermaphrodites gather into breeding aggregations during the reproductive season. The aggregations contain both mating and egg-laying animals, and are associated with masses of recently deposited egg cordons. Behavioral studies suggest that cordon-derived pheromonal factors are primarily responsible for establishing and maintaining the aggregations. Egg-laying animals are more attractive than sexually mature, but nonlaying, conspecifics and have a shorter mean latency to mating; egg cordons and egg-cordon eluates, when placed in the surrounding seawater, enhance the attractiveness of nonlaying animals and reduce their mean latency to mating. Similar effects are observed when extracts of the atrial gland are placed in the seawater, suggesting that secretory products of this oviductal exocrine organ may function as sexual pheromones. Biochemical analyses indicate that there may be multiple attractants in atrial gland extracts, and that at least one of these (A-NTP) is a peptide encoded by the A gene. The A gene belongs to a small family of structurally related genes that are expressed in a tissue-specific manner. Another member of the family, the egg-laying hormone (ELH) gene, is expressed in the neuroendocrine bag cells. Peptide products of the ELH gene act as neurohormones and nonsynaptic neurotransmitters, initiating egg laying and coordinating its associated behaviors. Peptide products of a family of genes may thus act internally and externally to coordinate both male and female reproductive activities.