Mortality in eiders (Somateria mollissima) on the Ythan estuary, Aberdeenshire, and elsewhere, has been claimed to be associated with the spiny-headed worm, Profilicollis botulus. Eiders acquire the disease by eating crabs (Carcinus maenas) infected with the cystacanth stage of the parasite. A detailed quantitative study was undertaken to elucidate some of the factors which control the abundance of the parasite in its intermediate and definitive hosts. During 1979-81 the P. botulus burdens in eiders and crabs were regularly monitored. Eiders were either taken alive off the estuary, or off the nest, and sacrificed, or found dead on the shore line. Crabs were caught in traps at two sites on the estuary. Female P. botulus in eiders were assigned to one of five categories according to the state of development of the eggs contained within their pseudocoelom. To estimate the size range and numbers of crabs eaten by eiders, crab chelipeds, or parts of chelipeds, were recovered from eider faeces collected monthly from roost sites. Regression equations relating certain cheliped dimensions to crab carapace width were used to estimate the size range eaten. The numbers of chelipeds per unit weight of faeces, equivalent to the quantity excreted per eider per day, was used to estimate the numbers of crabs eaten. Daily P. botulus infection rates in eiders were calculated by combining the quantitative data on crab intake by eiders with the cystacanth burdens in the crabs. Captive eiders, reared from eggs, were used to determine the age, days post-infection, of the five categories of female parasites. These development times were combined with the burdens found in the eiders sampled from the estuary to produce a static life-table of the mortality of female P. botulus. From this, estimates of the parasite's establishment rate, mortality rate and lifespan were obtained.