Abstract: The invention provides a 6-day Daphnia reproductive bioassay for detecting and confirming the presence of a toxic substance in an aqueous sample, and/or for screening the substance as an endocrine disrupter. According to the assay, a test sample is brought into contact with at least three adult, oviporous Daphnia of a single clone under conditions of crowding and suboptimal growth conditions to cause stress and stimulate sexual reproduction. The preferred clone for use in the assay is Daphnia galeata-mendotae Wingra clone CDF-1. The bioassay is based upon the measurement of five endpoints that convey quantitative information about the biological activity of the substance: survivorship, numbers of female offspring, numbers of male offspring, number of resting eggs, and number of offspring that display developmental deformities. Also provided are kits for use in conducting the bioassay.