Endocrine disrupting chemicals are a major class of pharmaceuticals that are known to influence sexual development, reproduction and endocrine systems in humans and other animals even at very low concentrations. 17a-ethinylestradiol (EE2) is one such chemical, a synthetic estrogen compound used in the contraceptive pill, EE2 is ubiquitous in surface waters globally. Where organisms may not be noticeably affected by a toxicant, they may still be incapable of functioning in an ecosystem if their regular behaviour is modified. In behavioural studies with fish, pollutants have been shown to influence behaviour at levels below those known to effect physiological endpoints, thus, behavioural ecotoxicology has gained popularity as a direct measure of xenobiotic exposure. In this study, a population of invasive eastern mosquitofish (Gambusia affinis) were taken from the wild and exposed to EE2 at environmental concentrations. G.affinis were exposed for 21 days to either control, 1, 3, 10 or 30ng/l EE2 then run through a series of behavioural assays which were video recorded. Boldness, foraging capacity, sociability, male mate choice and courtship behaviour were documented in individual fish, resulting in a set of behavioural data for 150 individual fish that provides a picture of overall behaviour for each individual. Video was analysed with an animal tracking software Ethovision®XT. At the end of the experiment, fish livers were dissected and a selection of genes corresponding to xenobiotic metabolism, stress and reproduction were investigated. The additional component of genetic analysis in this behavioural study allows for a more robust analysis into the mode of toxicity induced by EE2.