Although bacteriophage have been reported to be the most abundant organisms on earth, little is known about their contribution to the ecology of natural communities of their host bacteria. Most importantly, what role do these viral parasitoids play in regulating the densities of bacterial populations? To address this question, we use experimental communities of Vibrio cholerae and its phage in continuous culture, and we use mathematical models to explore the population dynamic and evolutionary conditions under which phage, rather than resources, will limit the densities of these bacteria. The results of our experiments indicate that single species of bacterial viruses cannot maintain the density of V. cholerae populations at levels much lower than that anticipated on the basis of resources alone. On the other hand, as few as two species of phage can maintain these bacteria at densities more than two orders of magnitude lower than the densities of the corresponding phage-free controls for extensive periods. Using mathematical models and short-term experiments, we explore the population dynamic processes responsible for these results. We discuss the implications of this experimental and theoretical study for the population and evolutionary dynamics of natural populations of bacteria and phage.