The sand goby (Pomatoschistus spp.) is a small estuarine fish. Its abundance, life history, and sedentary nature lead to its adoption as a key species in the U.K. Endocrine Disruption in the Marine Environment (EDMAR) Program. This study investigated the presence of classic markers of estrogenic exposure by determining vitellogenin (VTG) and zona radiata protein (ZRP) mRNA levels and ovotestis in estuarine-caught male gobies and investigated morphological changes in the urogenital papilla (UGP). Laboratory exposures to estrogens were also conducted to ascertain the responses of these markers. Wild-caught male fish showed no evidence of ovotestis, VTG, or ZRP mRNA induction. Laboratory exposures suggested that sensitivity of the goby to VTG/ ZRP mRNA induction was similar to flounder. The UGP inspection of wild-caught specimens revealed evidence of feminization of male papillae, a condition denoted as morphologically intermediate papilla syndrome (MIPS). Morphologically intermediate papilla syndrome was more prevalent at estrogenically contaminated sites. Juvenile goby experimentally exposed to 17beta-estradiol for 11 to 32 weeks exhibited signs of the MIPS condition, showing that it was inducible by estrogenic exposure and could therefore be a form of estrogenic endocrine disruption. The estuaries where the MIPS condition was most prevalent (>50% at certain sites) were the Tees, Mersey, and Clyde. The potential of the MIPS condition to significantly interfere with reproductive performance is discussed as well as its use as a monitoring tool for endocrine disruption in the estuarine environment.