Aims: The purpose of this study was to comprehensively compare the response of nine biosensors capable of being induced by Hg. Induction by Hg was based upon the insertion of merR, merB, zntA and zntR promoter genes. LuxCDABE or lucFF reporter genes expressed luminescence, and host organisms were Escherichia coli, Vibrio anguillarum and Pseudomonas fluorescens. The role of transcriptional switches, reporter mechanism and host organism was to be investigated. Methods and Results: All biosensors were subjected to the same assay conditions. Sensors had their own individual growth characteristics and response to the doses of Hg tested. Maximum bioluminescence response was induced by concentrations of Hg between 2.5 nM and 5 muM. E. coli pRB28 was found to detect levels of Hg as low as 1.6 nM and yet was capable of operating in a concentration range of up to 12.5 muM. Conclusions: The response of the sensors demonstrated their suitability for analysis under environmentally relevant concentrations. The sensitivity of the sensors, the optimum range and the expediency of the assay could not be related to a single sensor trait. It may be concluded that biosensor performance is dependent on more than one of the single factors studied. Significance and Impact of the Study: The results show that comparative testing of sensors is an important step in evaluating the relevance and performance of biosensors prior to routine environmental application.