Introduction The physiological effects of environmental acid stress on fish have been thoroughly reviewed in recent years (Muniz & Leivestad, 1980a; Fromm, 1980; Haines, 1981; Spry, Wood & Hodson, 1981; Brown, 1982; Leivestad, 1982; Wood & McDonald, 1982; McDonald, 1983a; Howells, Brown & Sadler, 1983; Howells, 1984); there are certainly not enough new data to justify yet another compendium. Instead, I will first summarize our current knowledge on the acute toxic mechanisms of pure acid stress to adult fish, and in so doing attempt to correct the widely held misconception that external water acidity must cause internal acidosis in the animal. By means of this brief summary, I hope to illustrate that external acidity has proven to be an exceptionally useful probe of normal physiological processes in freshwater teleosts. Secondly, I will describe some of our recent findings on the physiological responses to long term, low level acid stress, and acid-aluminium interactions, both of which may have greater ecological relevance than short term acid stress for ultimate survival in the wild. Acute responses to pure acid stress Background Relatively short term depressions to pH = 4.0–4.5, usually as a result of snowpack melt in the spring, or highly acidic runoff in the summer and autumn, have often been observed in natural soft waters of both northern Europe and eastern North America (e.g. Jeffries, Cox & Dillon, 1979; Harvey et al., 1981; Christophersen, Rustad & Seip, 1984; Marmorek et al., 1985).