SUMMARY.1. A procedure has been developed which uses environmental data to predict the probabilities of macro‐invertebrate taxa occurring at running‐water sites in Great Britain.2. Biological, physical and chemical data were collected from twenty‐ one sites on three river systems in order to evaluate the procedure.3. For most sites the number and type of taxa recorded, using a standard sampling programme, were very close to those predicted using twenty‐eight environmental variables.4. Comparison with other studies at the same sites showed that most taxa whose probability of occurrence was ≥0.5 could be found with more intensive sampling.5. Reducing the number of variables used in making the predictions from twenty‐eight to five resulted in only a slight loss of predictive accuracy.6. Combinations of chemical and physical variables gave better predictions than equivalent numbers of physical variables only but the latter may be more appropriate where chemical pollution is known, or suspected to occur.7. The procedure is of practical value in the detection and assessment of pollution.8. It may also be used to explore patterns in the structure and functioning of stream