A histological examination was carried out on a young harbor porpoise, Phocoena phocoena (♀),1.3m long, caught in a set net installed off Takashima, Otaru City, Hokkaido, facing the Sea of Japan. The formalin-preserved left lobe of the ovary was 13mm long and 0.2g in weight, the right lobe being 20mm and 0.2g, respectively. Both lobes were smooth, flat and disc-shaped, and revealed similar microscopic conditions which confirmed the young age of the animal. The cortical region comprised a number of primordial- and primary follicles, and few secondary follicles, whereas the medullary region, consisting of a large amount of interstitial tissue, included a few vesicular and many atretic follicles, in association with spiral arteries. However, no Graaffian follicles were encountered, and the uterine glands were simple and underdeveloped.