Profilicollis botulus (Van Cleave, 1916) is a common intestinal parasite of eider ducks which is transmitted by the shore crab. The parasite occurs more frequently, and in larger numbers, in juvenile birds than in adults. In the latter there is a significant correlation between increasing host age and decreasing worm burdens. The infections in both eider ducks and shore crabs were investigated for 12 consecutive months. The incidence of infection (number of birds infected) was high in both adult and juvenile birds but declined in winter. The intensity of infection (number of worms per infected bird) was significantly higher in juvenile birds, the peak occurring in their first summer and autumn, with a smaller peak in April of the following year. Adult birds showed peaks in intensity of infection in October and April, with a decline in males only during the winter. Most of the worms were found in the third quarter of the intestine (measured from the junction with the gizzard to the rectum). The intensity of infection with the cystacanth in shore crabs increased with the size (age) of the host. An approximate index of abundance of crabs and of the cystacanths contained therein, indicated a mid-summer peak of availability which coincided with the developing infections in young eiders. The time of minimum availability of crabs and the lowest level of infection coincided with the drop in infection rates in the definitive host. Natural worm loss from eiders was demonstrated by examination of faecal specimens collected on the Ythan estuary. Feeding trials, with captive birds and known food items, revealed a preference for crabs, although this was not confirmed by field observations. It was concluded that P. botulus is an infection, primarily of juvenile birds, which is acquired early in the first year of their life. A regular but probably small intake of crabs, in relation to the total food consumption, would be sufficient to maintain the level of parasitism seen in the birds.