Two papers published in peer-reviewed journals (Holt, 2006; Holt et al., 2006) describe the mathematical basis of the UK Non-Native Organism Risk Assessment Scheme2. The current risk assessment methodology (specifically including the proposed scoring system and the categorization of responses) has also been peer reviewed in a report by RPS Group Ltd, available on the website of the GB Non-Native Species Secretariat3. In the notes that follow, some previously unreported problems with the mathematical analysis on which the Scheme is based are outlined