The influence of specific phages on the population size of their host bacteria was studied using a model system of a selected phage and its host bacterium, identified as Aeromonas sp. Isolated from a lagoon in the southern Baltic Sea. For all initial phage/host ratios (PBR [phage/bacterium ratio] of 0.1 to 100), phage and host abundances reached a ratio of about 5 to 50 within the first 8h Interestingly, this range of phage/bacterium ratios of a single phage-host system corresponds to the ratio of virus to bacterial abundance found in natural habitats. The proportion of sensitive cells containing mature phage particles increased in the first 4h to 40%, and then decreased (at rates dependent on the initial ratio) within 12h (initial PBR of 100), 48 h (initial PBR of 10 and 1) and 72h (initial PBR of 0.1) to less than 1%. There was evidence for rapid development of immunity (probably pseudolysogeny), especially at high initial phage concentrations. After 96h incubation, carbon, nitrogen and phosphorus were added to improve the nutritional state of the bacteria. Although nutrients were added to all treatments when phage/host ratios were nearly equal, quite different reactions were observed for the bacteria populations with different initial PBRs. Those with low initial PBRs showed the most pronounced increase.