The community composition and structure of planktonic heterotrophic bacteria (903 isolates) sampled from a small eutrophic lake in northern England (Priest Pot) was studied with respect to season (four samples) and depth (to 3.1 m). Bacteria (887) were isolated on tryptic soy broth agar and identified to 48 genera using fatty acid methyl ester analysis. The two most abundant genera isolated were Aeromonas and Pseudomonas which, respectively, dominated the middle to bottom depths in August and all depths in February. The structure of the sampled community was described using: species richness, Simpson's index and the Shannon-Wiener index. All three indices detected a number of significant differences with depth demonstrating stratification. The greatest stratification of the bacterial community was observed in August when bacterial counts correlated strongly and negatively with diversity. Using structural measures was found to be preferable to the use of species frequencies in the analysis of perturbation and succession in community structure. Insensitivity to one or more of eight antibiotics was observed in 71% (61/86) of the isolates tested particularly in Gram-negative genera. Bacteriocinogeny and lysogeny was observed in 36% (32/90) of isolates. Using sensitive indicator strains, two of 10 producing strains produced virus, while the others produced bacteriocins.