Structural changes at synapses are associated with long-term facilitation (LTF) of synaptic transmission between sensory and motor neurons in Aplysia. We have cloned a cDNA encoding Aplysia adducin (ApADD), the Aplysia homolog of mammalian adducins that are regulatory components of the membrane cytoskeleton. ApADD is recovered in the particulate fraction of nervous system extracts and is localized predominantly in the submembraneous region of Aplysia neurons. ApADD is phosphorylated in vitro by protein kinase C (PKC) at a site homologous to the in vivo PKC phosphorylation site in mammalian adducins. Phosphorylation of ApADD at this site is also detected in vivo in the intact Aplysia nervous system and is increased 18 hr after serotonin-induced LTF. In contrast, there is no change in phosphorylation during short-term facilitation or 1 hr after initial LTF induction. Thus, ApADD is modulated specifically with later phases of LTF and provides an attractive candidate protein that contributes to structural changes accompanying long-lasting synaptic alteration.