Loch Fleet is an oligotrophic upland lake in Galloway, south‐west Scotland. It once supported a brown trout,Salmo truttaL., sport fishery with low annual catches (<150 fish year−1) but catches declined markedly after 1950 and no fish were caught after 1975. Diatom records for the lake sediments indicate acute acidification since 1975, pH changing from c. 5–8 to 4–6.In 1984 a project was set up at Loch Fleet to investigate techniques of acidity mitigation, with a view to restoring fisheries in this and similarly affected waters. An underlying assumption of the project was that fish had been lost as a direct result of acidity and associated factors. Studies were therefore undertaken during a 2‐year baseline period (1984–1986) to validate this assumption.Fish surveys using a variety of techniques (gill‐netting, trapping, electrofishing) confirmed that trout were absent from the Loch and its afferent streams, and also from its main outlet stream, the Little Water of Fleet, for a distance of 7 km downstream. Trout were present below this point but are prevented from passing upstream by a 5‐m waterfall. Eels,Anguilla anguillaL., were present throughout the Little Water of Fleet, though not in the Loch itself. Population densities of both species were low, with less than 7 eels and 5 trout per 100 m2.Survival studies using brown trout ova and yolk‐sac fry indicated that conditions in the Loch and its afferent streams were acutely toxic to these stages as a result of the low pH (pH 4.5), low calcium (I mg l−1) and high aluminium concentrations (200 μg 1−1total Al, 60 μg 1−1inorganic monomeric Al). Trout fingerlings could survive these conditions in short‐term tests (9 days) but, in chronic exposure tests lasting up to 180 days carried outin situin streams adjoining the loch, no fish survived this period. This toxicity was eliminated in experiments where pH was raised to 5.4 by KOH addition.It is concluded that the loss of the brown trout fishery at Loch Fleet occurred as a direct result of acidity and related factors, probably acting in the first instance on the sensitive intra‐gravel ova and yolk‐sac fry stages,