The distribution and morphology of gut endocrine cells, which are immunoreactive to crustacean hyperglycaemic hormone (CHH) and the corresponding precursor-related peptide (CPRP), have been described in the shore crab Carcinus maenas. The cells are uniquely distributed throughout the fore- and hindgut, but were never observed in the midgut or associated caeca. Expression of CHH and CPRP in the gut endocrine cells is generally restricted to premoult, although small numbers of immunoreactive cells were observed in intermoult and postmoult. A notable feature of the distribution of these slender cells was that, whilst they are distributed evenly over much of the fore- and hindgut, all extrinsic and intrinsic muscles of the gastric and pyloric stomach examined were surrounded by a ring(s) of cells, suggesting a mechanoreceptive function. Ultrastructural studies revealed that these cells contain numerous immunopositive, electron-dense granules. This suggests that they are "paraneurones", which secrete CHH and CPRP into the haemolymph during ecdysis, accounting for the ecdysial surge in CHH, which is implicated in water uptake and swelling prior to ecdysis.