Serotonin (5-HT) plays important roles in various behavioral and physiological processes in Aplysia californica. These include feeding, locomotion, circadian rhythm, learning and memory, synaptic plasticity, and synaptic growth. Serotonin modulates these various functions by interacting with different 5-HT receptor subtypes that are coupled to various second-messenger systems. We report here the isolation and characterization of the first two serotonergic receptors from Aplysia californica, Ap5-HTB1 and Ap5-HTB2, using a strategy based on the amino acid sequence homology among G-protein-coupled biogenic amine receptors. Ap5-HTB1 and Ap5-HTB2 are both intronless and highly homologous to each other, sharing 79.5% sequence identity at the amino acid level. Sequence comparison reveals that these receptors are 33.1 to 23.3% identical to the following 5-HT receptors: 5-HTdro1 > 5-HT6 > 5-HTlym > mouse 5-HT1B > 5-HTdro2A > mouse 5-HT7 > rat 5-HT2A. Both Ap5- HTB1 and Ap5-HTB2 encode functional 5-HT receptors. When expressed in cultured cells, these receptors stimulate phospholipase C in response to 5-HT in a dose-dependent manner. This stimulation can be blocked by specific 5-HT receptor antagonists. Using RT-PCR and Western blot analysis, we have detected these receptors in the CNS (Ap5-HTB2) and in the reproductive system (Ap5-HTB1). The nucleotide sequences of Ap5- HTB1 and Ap5-HTB2 were submitted to GenBank; the accession numbers are L43557 and L43558, respectively.