The nervous system of Aplysia californica has three isoforms of protein kinase C (PKC): the conventional PKC Apl I, the novel PKC Apl II, and the atypical PKC Apl III. Each isoform has distinct requirements for activation and distinct downstream roles in synaptic plasticity. PKCs can be cleaved by calpains into constitutively active forms, called protein kinase Ms (PKMs). Multiple forms of plasticity in Aplysia are mediated by PKMs, and these may be due to cleavage of distinct isoforms of PKC. PKCs also interact in complex ways with other second messenger pathways. The diversity of PKC isoforms allows for this family of kinases to play important roles in decoding extracellular stimuli into the formation of distinct molecular memory traces.