At the request of the Bisphenol A Global Industry Group (representing The Society for the Plastics Industry and CEFIC), a study was undertaken to determine the effect of chronic exposure to Bisphenol A (BPA) on sex differentiation in larvae of the African clawed frog (Xenopus laevis). This request was in response to a study published by Kloas et al. (1999) (Ref 1) which reported alteration of sex ratio (feminisation) in Xenopus larvae exposed to BPA at a nominal concentration of 23 p.g I . The current study incorporated a number of improvements in study design, including greater replication, increased statistical power, analytical monitoring of exposure concentrations and use of a flow-through exposure system to maintain steady concentrations of the test substance in the test vessels. Other changes to the conduct of the study included constant monitoring of temperatures in two of the test vessels, regular measurements of temperature, pH and dissolved oxygen concentration in all test vessels, in situ fixation of gonadal tissue to aid in assessment of gross gonadal morphology, and use of different statistical methodology in analysis of sex ratios. We maintain that these alterations constituted improvements in terms of better water quality data, more reliable assessment of gonadal sex, and more appropriate treatment of sex ratio data, enabling a more robust interpretation of the results. Moreover, these improvements did not result in changes to the fundamental design and endpoints of the. study, and comparisons of the findings of this report to those of the original study reported by Kloas are therefore valid. The definitive study was run between 10 March and 8 June 2000. The study number was AF0072/A and the Brixham test substance number was AF0072. The sample was supplied by the Shell Research and Technical Centre and the reference number was ST98148. The sample of 1715-estradiol, used as a positive control (PC) for oestrogen agonist effects, was obtained from Sigma Chemicals (Lot 77HO666) and the Brixham test substance reference number was AFO314. All original data, together with other relevant records, are filed in the Brixham Environmental Laboratory archive.