Histopathological and mycological studies were made on mycotic gastritis in juvenile ayu, Plecoglossus altivelis. The disease occurred 5 days after the transportation of fish from a hatchery pond to two rearing ponds of Tochigi Prefectural Fisheries Experiment Station, Tochigi, Japan. The pyloric region of the stomach was red. Numerous fungal hyphae penetrated into the stomach wall. Seven fungal isolates were obtained from the lesions. Three isolates were identified as Saprolegnia diclina Type 1. Based on histology, hyphae were most numerous in the pyloric stomach, but also penetrated abdominal adipose tissue, the pancreas, spleen, kidney, swim bladder, gonad, and liver. In some cases, the hyphae penetrated trunk muscles.