Objective: To develop and test a new singleitem physical activity screening tool, suitable for assessing respondents' eligibility for behaviour change interventions. Design: Two single-item assessment tools were developed, one using a "past week" recall period, the other using a "past month" recall period. A quota sampling system was used to recruit 480 adults from across England, Scotland and Wales. Half the sample completed the past-week question and half completed the past-month version. Test-retest reliability was assessed over a 2- to 5-day period. Concurrent validity was assessed using the Global Physical Activity Questionnaire and the UK Active People Survey. All surveys were completed via telephone interviews. Results: Both versions of the single-item instrument demonstrated strong reproducibility (r=0.72-0.82), using Spearman's rank correlation coeffi cients. The past-week recall question showed strong agreement in the classifi cation of respondents meeting the current physical activity recommendation (kappa=0.63, 95% CI 0.54 to 0.72).Concurrent validity over the past week compared to the Global Physical Activity Questionnaire was modest (r=0.53) and slightly weaker for the past month compared to the Active People Survey (r=0.33-0.48). Conclusion: Both versions of the new single-item measure performed as well as other short physical activity tools in terms of reliability and concurrent validity. Criterion validity testing of the single-item measure is recommended to establish its ability to assess objectively measured physical activity levels. In addition, further research to assess the responsiveness of the single-item measure in detecting changes in physical activity will inform its usefulness in programme evaluation.