Mechanisms controlling virus abundance and partitioning of loss of bacterial production between viral lysis and protozoan predation are discussed within the framework of an idealized Lotka-Volterra–type model. This combines nonselective protozoan predation with host-selective viral lysis of bacteria. The analysis leads to a reciprocal relationship between bacterial diversity and viruses, in which coexistence of competing bacterial species is ensured by the presence of viruses that ‘kill the winner,’ whereas the differences in substrate affinity between the coexisting bacterial species determine viral abundance. The ability of the model to reproduce published observations, such as an approximate 1:10 ratio between bacterial and viral abundance, and the ability of viral lysis to account for 10–50% of bacterial loss are discussed.