Pleistophora ovariae infects ovarian tissue of golden shiners (bait fish), resulting in sterility. It is an important disease in the bait-fish industry. The organism has no intermediate host and is transmitted horizontally (through ingestion of infective spores) or vertically (through infected ova). Fertility declines as fish age, eventually resulting in sterility. Grossly, infected ovarian tissue appears marbled. The diagnosis is confirmed by examination of a wet mount of suspect tissue, revealing the presence of microsporidian spores. Although there is no treatment, the problem can be managed by discarding female broodstock when they reach 1 yr of age and replacing them annually. Although the young females remain infected, the disease is not yet sufficiently advanced to have a major impact on fertility.