Freshwater fisheries, which generate more than 30 billion dollars per year in the United States, are critically important resources.  Regional-scale assessments of freshwater fisheries are difficult to perform, however, as most studies have been conducted on discrete populations.  Here, I present a simple model that can be used to predict fish densities at very large spatial scales.  The model first assumes that energetic resources are transferred among trophic levels at a predictable rate (10% transfer efficiency).  It then uses the scaling relationship between body mass and abundance (-3/4 power) to predict the densities of fishes within higher trophic levels, at any spatial scale of interest.  Model predictions were evaluated by comparison with empirical data from four warm-water and four cold-water systems.  Predicted densities were within +/- 50% of the empirical densities in six of eight systems, and were within an order of magnitude in all systems.  Also, when sensitivity analyses were used to examine alternative parameter values, I found no evidence of a systematic bias in the model predictions.  I therefore conclude that the model is a useful means of predicting fish densities at large scales, in a quick and efficient manner.