The development of the Western Flower Thrips (Frankliniella occidentalis Pergande; Thysanoptera: Thripidae) was studied at six temperatures between 10 and 35°C. Developmental rate increased linearly as rearing temperature increased. It was estimated that 268 degree-days, above a threshold temperature of 7.9°C, were required to complete development from egg to adult. These data were related to records of field temperatures in the West Midlands region of the UK, to estimate the potential number of generations per year that could complete development in outdoor conditions. Using this data, a maximum of between three and five generations could have developed annually between 1986 and 1995, (in the absence of factors impairing continuous development). The application and relevance of this data as an indicator of the potential range of F. occidentalis is discussed.