The effects of the inability of suspension-feeding protozoa to collect bacteria over the whole range of sizes in the bacterial size distribution were examined by constructing mathematical models based on this assumption. Systems of suspension-feeding protozoa grown on both growing and nongrowing bacteria were examined in both batch and continuous culture. The models were able to predict three experimental observations common in such systems. Some additional features of the systems which should be useful in interpreting results of experiments with suspension feeding protozoa and in designing new experiments were predicted, also.