The somata of the first FMRF amide-like immunoreactive (Fa-LIR) cells appear early during neurogenesis and are located posteriorly within the embryo. By the time that Aplysia californica hatches into a free-living veliger, it already contains 3 intensely immunoreactive cells with fibres which project anteriorly to cross the cerebral commissure and invade the developing pedal ganglia. Fa-LIR elements are also present by 30-33% of embryonic development in Lymnaea stagnalis, which hatches as a fully metamorphosed juvenile. The cells in L. stagnalis, are similarly located posteriorly in the embryo and have fibres which project to the cerebral and pedal ganglia. These findings suggest that such Fa-LTR cells are among the very first to develop within the gastropod nervous system and that they may be play important roles in subsequent neuronal development.