Signal crayfish (Pacifastacus leniusculus) have become increasingly widespread in rivers in Great Britain since their introduction in the late 1970s, causing extensive losses of indigenous white-clawed crayfish and negative impacts on communities of aquatic plants, invertebrates and benthic fish. Angling interests are increasingly concerned about possible impacts of signal crayfish on brown trout, sea trout (Salmo trutta) and Atlantic salmon (S. salar). This study of a limestone headwater stream in the Pennine uplands, Yorkshire, compares density of fish and two species of crayfish in two years. Signal crayfish are progressively replacing white-clawed crayfish. Surveys showed a significant negative relationship between the fish and signal crayfish. Sites with white-clawed crayfish (1–2 crayfish/trap night) had abundant juvenile trout (> 47·100 m−2). Signal crayfish reached higher abundance (4–8 crayfish/trap night) and those sites had fewer fish (0 − 18.8·100 m−2). The signal crayfish population will expand to other tributaries over time. If similar reduction of salmonid recruitment occurs in those streams, there is potential for significant impacts on an important recreational fishery.