Estrogenic chemicals found within wastewater treatment work (WwTW) effluents have been shown individually to inhibit reproduction in fish, but the impact of the WwTW effluents themselves and the complex mixtures of environmental estrogens and other endocrine disrupting chemicals they contain has not been established. In this investigation, the effect of exposure to three WwTW effluents, with differing levels of estrogenic activity, was assessed on egg production in pair-breeding fathead minnow. Exposure to two of the three effluents tested resulted in a reduced egg production (by 28% for effluent I at a dilution of 50% and by 44% for effluent III at full strength), which was proportional to the estrogenic content of the effluents. The test effluents, however, had a greater effect on egg production than might be expected, on the basis of both the response they induced for induction of vitellogenin (an estrogen exposure biomarker) and when compared with an equivalent estrogen exposure to EE2. These data show that reliance on relatively simple biomarker responses for estrogenic activity alone, such as vitellogenin, can significantly underestimate the impacts of estrogenic WwTW effluents on fitness parameters such as reproductive health that are regulated by more complex estrogenic (and other endocrine) signaling mechanisms.