Data are presented from a 10-year (1984 to 1993) study of a Salmo trutta population in the Afon Cwm, a small tributary of the Afon Dyfi, mid-Wales. The stream is a spawning and nursery area for sea trout. Growth of trout within the stream can be summarized by a von Bertalanffy growth coefficient (K) of 0·310, with asymptotic length (1∞) 21·6 cm and with length at age 1 of 7·6 cm. Mean population density in the whole stream varied from year to year between 0·05 and 0·60 0-group trout m−2 and between 0·05 and 0·70 older trout m−2. Mean biomass varied, between years, from 0·1 to 3·5 g m−2 for 0-group and from 1·3 to 10·4g m−2 for older trout. Loss between 3 and 5 months of age appeared to be proportionate at about 50 to 60% and instantaneous loss rate from 5 to 53 months of age varied from 0·04 to 0·10 month−1 and was positively correlated with cohort number at 3 months of age. Production between 3 and 53 months of age varied between cohorts from 3 to 8 g m −2 live weight.