Between April 3 and September 24, 1991, the concentrations of bacteriophages infecting bacterial strains, isolated in 1990 and during this investigations, were determined in 35 samples of seawater taken at station ‘Kabeltonne’ adjacent to Helgoland. Similar to the findings of 1990, phage concentrations of several hundred plaque forming units (PFU) ml−1 were observed with a number of indicator strains, the maximum concentration being at least 1.5×103 PFU ml−1. These high concentrations lasted for only a few days, generally decreasing at rates between 0.6 and 0.9 day−1. Phage concentrations of 0 to 2 PFU ml−1 were found to be predominant until the end of June, occasionally attaining 5 PFU ml−1. From July through September, when high phage concentrations were observed with some indicator strains, between 0 and 10 PFU ml−1 were found in the majority of tests. As revealed by a final phage-host cross-reaction test, the greater part of 138 indicator bacteria is genetically related, and almost half of the 200 phage strains tested are propagated only by their original indicator bacterium. The possible importance of mutational events for the maintenance of phage-host systems in nature is discussed.