Vitellogenin, a yolk protein produced in the liver of oviparous animals in response to estrogens, normally occurs only in sexually mature females with developing eggs. However, males can synthesize vitellogenin when exposed to environmental estrogens, making the abnormal production of vitellogenin in male animals a useful biomarker for xenoestrogen exposure. In 1997-2001, as part of the Washington States Puget Sound Assessment and Monitoring Program (PSAMP), we surveyed English sole from a number of sites for evidence of xenoestrogen exposure, using vitellogenin production in males as an indicator. Significant levels of vitellogenin were found in male fish from several urban sites, with especially high numbers of fish affected in Elliott Bay, along the Seattle Waterfront. Intersex fish were rare, occurring in only two fish out of more than 2,900 examined. Other ovarian and testicular lesions, including oocyte atresia, were also observed, but their prevalence did not appear to be related to xenoestrogen exposure. However, at the Elliott Bay sites where abnormal vitellogenin production was observed in male sole, the timing of spawning in both male and female English sole appeared altered. Sources of xenoestrogens and types of xenoestrogens present in Elliott Bay are unknown, but may be associated with industrial discharges, surface runoff, or combined sewer outfalls.