The ovarian histology of 140 sea otters (4 juvenile and 136 sexually mature) is described. Multilayered germinal epithelium occurs in thickened areas on the ovarian surface and in fissures which are simple or complexly branched. Occasionally an epithelial tube extends from the fissure into the cortex. Typically a single Graafian follicle reaches the prevulatory stage while others become atretic. Interstitial gland cells of theca interna origin are abundant and apparently secretory during estrus. The corpus luteum of preimplantation pregnancy has a medium to large antrum which is obliterated by the time the blastocyst implants. During delayed implantation, the luteal cells progressively hypertrophy and by the time of implantation they are polygonal with a uniformly granular, nonvacuolated cytoplasm. Following implantation, many small secondary cavities or spaces are observed in the corpus. Subsequently, they coalesce to form larger ones. These cavities are strikingly well developed during mid gestation. After parturition, the corpus luteum degenerates rapidly. A corpus albicans persists for at least two years before blending with the stroma.