Distribution and morphology of putative nitroxidergic (NO-ergic) cells were studied in the labial palps, esophagus, stomach, digestive gland, and intestine of the bivalves Crenomytilus grayanus, Modiolus modiolus, and Mytilus coruscus using NADPH-histochemistry (Hope & Vincent, 1989). NO-producing elements were found in all examined organs and regions of the digestive system. NADPH-diaphorase-(d)-positive staining was readily detectable in neurons, secretory cells, brush border epithelial cells, and in the nervous plexuses. Intraepithelial nerve cells were the most common NADPH-d-positive cell type throughout the digestive tube. These cells had a fusiform perikaryon, which gave rise to a thin apical process that extended toward the gut lumen, whereas the basal process contacted the basiepithelial NADPH-d-positive plexus. In the major typhlosole of C. grayanus, these cells constituted up to 1.28% of the total number of epithelial cells. The bivalve species studied exhibit a similar distribution pattern of NADPH-d-positive cells, which lie separately or form small groups of two to three in the basal part of the epithelium, being most abundant in the dorsal and ventral regions. Subepithelial NADPH-d-positive neurons were found in C. grayanus and M. modiolus. The labial palps, lips, and esophagus contained an abundance of intraepithelial and subepithelial secretory cells that stained positive for NADPH-diaphorase. Moreover, NADPH-diaphorase was detected in brush border epithelial cells of the primary and secondary ducts of the digestive gland. All examined regions of the digestive system contained basiepithelial and subepithelial NADPH-d-positive nerve fibers that formed loose to highly developed plexuses. The most prominent plexuses were found in the lips, esophagus, and intestine of the species studied.