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Our goal was to use novel fibreoptic sensors to make the first direct PCO2 measurements in the digestive tracts of live freshwater fish (anaesthetized, artificially ventilated, 12 degrees C). PCO2 levels in gastrointestinal fluids were substantially higher than in blood, and were elevated after feeding. In the carnivorous, gastric rainbow trout, the mean PCO2 in various parts of the tract increased from 7-13 torr (1 torr = 0.1333 kPa) during fasting to 20-41 torr after feeding, relative to arterial levels of 3.5-4 torr. In the agastric, omnivorous goldfish, the mean gut levels varied from 10-13 torr in fasted animals to 14-18 torr in fed animals, relative to arterial levels of 5-7 torr. These elevated PCO2 values were associated with surprisingly high HCO3- concentrations (greater than 40 mmol l(-1)) in the intestinal chyme. Incubations of food pellets with acid or water revealed endogenous PCO2 generation sufficient to explain gastric PCO2 in fed trout and anterior intestine PCO2 in fed goldfish. The impacts of possible equilibration with venous blood draining the tract are assessed. We conclude that fish are already coping with PCO2 levels in the internal gastrointestinal environment many-fold greater than those of current concern in the external environment for climate change and aquacultural scenarios.