The detrimental effect of oxygen depletion on fish is well known. Here a quantitative measure of anoxia is compared with the number of fish species in 52 south-central Ontario lakes. The anoxic factor (AF, days per year or per season) estimates the number of days that a sediment area equal to the lake surface area is overlain by anoxic water. It can be expressed separately as summer or winter AE Stepwise multiple-regression models were constructed with AF and common physical and chemical variables to predict fish species richness. Because some of the parametric regression assumptions were violated, the traditional statistics were evaluated with randomization tests to confirm significance. The best regression models included summer AF or lake surface area, or winter AF and mean depth, for subsets of acidic and circumneutral lakes; these models explained up to 75% of the variation in fish species richness. One of the models can be used to estimate the likelihood of winterkill. Anoxic factor was also correlated with the number of coldwater fish species present. The concept of AF can be used to model the distribution and number of fish species in stratified lakes and should be useful in the management of oxygen-dependent aquatic animals (e.g., finfish and shrimp) in freshwater and marine habitats.