The growth and mortality of roach (Rutilus rutilus), bleak (Alburnus alburnus), dace (Leuciscus leuciscus) and perch (Perca fluviatilis) in the River Thames at Reading are described. Walford plot constants were obtained, for year I-VI roach: L = 16.5 cm, k = 0.74 and the y-axis intercept = 4.3 cm, for bleak; L= 15.75 cm, k = 0.745 and the y-axis intercept = 4.0 cm, for year 0-VII dace; L = 20.0 cm, k = 0.82 and the y-axis intercept = 3.6 cm. The constants did not apply to older roach and dace since they grew slightly faster than expected. The growth in length of perch was more or less arithmetic once the fish matured and the Walford constants were not very indicative. In comparison with the growth of fish in other habitats, roach, dace and young perch showed a slow rate of growth which was attributed to overcrowding. Bleak and older perch grew relatively fast and the growth of older roach and dace showed some improvement. It was suggested that specialization of diet might prove to be the reason for this. Mortality and growth were such that the maximum biomass of a brood was reached at an early age and small size in roach (year V, 13.0 cm), dace (year V or under, less than 14.0 cm) and perch (year HI or under, less than 12.0 cm). It was evident that large populations of fish with slow growth rates were using nearly all their available food resources for maintenance rather than growth and that one method of improving growth would be to reduce the fish stocks.