The epidemiology of the sucking lice Echinophthirius horridus was studied over a 4 year period in which their harbour seal hosts experienced marked inter-annual changes in food availability. Prevalence and intensity of infection varied in relation to host age but not sex. Burdens were highest on immature seals, but both prevalence and intensity of infection were significantly higher in years when food availability was low. Observed intra-population variations in ectoparasite dynamics suggested that reported geographical differences in the prevalence of E. horridus may have resulted from methodological differences. In contrast to previous studies of other pinnipeds, lice were also absent on weaned pups, suggesting that E. horridus is transferred horizontally. No significant differences were found in the haematological parameters of infected and uninfected hosts. Nevertheless, there was a significant negative correlation between intensity of infection and several erythrocyte parameters, suggesting that high burdens of lice may compromise diving ability.