Patterns of community and population diversity are likely to be dependent on interactions between ecological variables. Here we address how two important ecological variables – extrinsic periodic mortality events (disturbances) and the presence of obligate-killing parasites – interact to affect the diversity of niche-specialist genotypes in laboratory populations of the bacterium Pseudomonas fluorescens. Consistent with previous studies, diversity was maximized at intermediate frequencies of disturbance in the absence of parasitic bacteriophages (phages). By contrast, no relationship was found between diversity and disturbance frequency in the presence of phage. The results can be explained in part by differential effects of phage on bacterial densities, and hence resource competition, under different disturbance regimes.