Abstract.1. Regular samples of flies swarming around cattle in Northumberland, England, were collected from late May to early October 1980–82, during weather judged to be suitable for fly activity.2. Results were grouped into weekly periods and averaged to reduce variation caused by the weather.3. For all species of Muscidae that were caught in reasonable numbers, the changes in abundance through the season are described.4. The proportion of each species in the total sample is discussed; biting species were not sampled as efficiently as were non‐biting species.Hydrotaea trritansandMorellia simplexwere the most numerous species forming 49% and 32% of the catch respectively.5. The total numbers of flies were low at the beginning of the sampling period, rose to a maximum in mid‐July, stayed high until mid‐September and then declined by