Gill number 8 of intermoult male Carcinus maenas (L.) (Decapoda: Brachyura) was used to study the morphological variation and distribution of the major gill cells. Three gill regions (proximal, mid and distal) were sectioned and examined by light and transmission electron microscopy. The ultrastructure of the three epithelial cell types (striated, chief and pillar) was similar to previously published descriptions, but some variations in cell form and distribution were observed. In all gill regions, striated and chief cells were larger towards the lamellar raphe than near the marginal canal. In the proximal gill region, the epithelium consisted almost exclusively of striated cells, implying an osmotic/ionic role for this region. In the mid gill region, the epithelium comprised a mixture of chief and striated cells. In the distal region, chief cells predominated, implying a respiratory role for this region. Pillar cells exhibited no clear distributional pattern within the gill and, although their shape varied with position, their involvement in ionic regulation, osmoregulation, respiration, structural support or the flow of haemolymph through the gill is not clearly defined. Nephrocytes, thought to be involved in gill defence mechanisms, occurred in the haemolymph space or attached to the intralamellar septum and showed no structural variability. Glycocytes were always associated with the intralamellar septum, implying that there are specific gill regions for glycogen storage. Haemocytes showed some structural variation but there was no obvious correlation with gill region; small-granule haemocytes were the most common haemocyte type in the gill.