Hypoxia is a common occurrence in aquatic habitats, and it is becoming an increasingly frequent and widespread environmental perturbation, primarily as the result of anthropogenic nutrient enrichment and climate change. An in-depth understanding of the hypoxia tolerance of fishes, and how this varies among individuals and species, is required to make accurate predictions of future ecological impacts and to provide better information for conservation and fisheries management. The critical oxygen level (P-crit)m has been widely used as a quantifiable trait of hypoxia tolerance. It is defined as the oxygen level below which the animal can no longer maintain a stable rate of oxygen uptake (oxyregulate) and uptake becomes dependent on ambient oxygen availability (the animal transitions to oxyconforming). A comprehensive database of P-crit values, comprising 331 measurements from 96 published studies, covering 151 fish species from 58 families, provides the most extensive and up-to-date analysis of hypoxia tolerance in teleosts. Methodologies for determining Pcrit are critically examined to evaluate its usefulness as an indicator of hypoxia tolerance in fishes. Various abiotic and biotic factors that interact with hypoxia are analysed for their effect on P, including temperature, CO,, acidification, toxic metals and feeding. Salinity, temperature, body mass and routine metabolic rate were strongly correlated with P-crit; 20% of variation in the P-crit, data set was explained by these four variables. An important methodological issue not previously considered is the inconsistent increase in partial pressure of CO, within a closed respirometer during the measurement of P-crit. Modelling suggests that the final partial pressure of CO, reached can vary from 650 to 3500 mu atm depending on the ambient pH and salinity, with potentially major effects on blood acid-base balance and P it itself. This database will form part of a widely accessible repository of physiological trait data that will serve as a resource to facilitate future studies of fish ecology, conservation and management.