The population ecology of the diseases of the common dab (<I>Limanda limanda</I> L.) has been studied and the use of fish disease as a monitor of pollution in the marine environment reassessed. The diseases included in this study were lymphocystis disease, hyperplasia/papilloma, ulcer disease, inflammatory fat-cell necrosis, and X-cell disease. The interactions responsible for determining disease abundance were assessed by carrying out a detailed longitudinal field survey and interpreting the observed changes in abundance by reference to the population biology of the common dab, the abiotic environment, and the known biology of the diseases. The diseases were not evenly distributed within the host population. Significant differences in prevalence occurred between sexes, ages, and fish of differing maturity status. These were interpreted in terms of varying susceptibility of the host, the response of the host to infection and the dynamics of the diseases. Analysis of the age-prevalence relationships suggests that although some diseases have the potential for their abundance to be regulated this is not fulfilled. The temporal distribution of the diseases consisted of long-term trends, seasonal cycles, and short-term fluctuations. A variety of potential causal factors were highlighted including the spawning behaviour of the dab, temperature, and bacterial abundance related to environmental productivity. The patterns in the spatial distribution of the diseases were reflected in variation in prevalence on both large (˜100nml) and small scales (˜1nml). In this case the causal factors highlighted were salinity, host population density, and again bacterial abundance related to productivity. No effect of pollution on disease prevalence was demonstrated. Neither could the occurrence of lymphocystis disease be related to the concentration of chemical contaminants in the livers of dab.