Abstract.A detailed histopathological study of naturally occurring and experimentally induced outbreaks of nephrocalcinosis was carried out on rainbow trout from a number of farms.Histopathological effects were confined to the kidney, stomach and skeletal muscle. Lesions of the stomach wall were often found in the absence of renal lesions but muscle lesions were only found in the presence of severe renal damage. It was considered that the calcification was the result of deposition of calcium salts within necrotic tissue, and thus was dystrophic rather than metastatic. Variation between farms was considered to be due to variations in the effects of different aetiological factors in the complex series of physico‐chemical events which appear to induce the conditio