Increased mortality in a flock of non-breeding mute swans (Cygnus olor) on a Scottish loch was investigated. Postmortem examinations were carried out on eight adult and six immature swans. The commonest cause of death, found in eight birds, was lead poisoning associated with the ingestion of large lead fishing weights. Heavy parasitic burdens were found in five immature birds, involving combinations of the gizzard worm Amidostomum species, the thornyheaded worms Polymorphus minutus and Profilicollis anatis, and the tracheal trematode Orchipedum tracheicola. Other parasites of lesser significance were the biting louse Trinoton anserinum, the tapeworm Wardoides nyrocae, the hairworm Capillaria species and the intestinal trematode Echinoparyphium recurvatum. Eight of the 14 swans carried trematodes of the family Schistosomatidae, which may be involved in human cercarial dermatitis or ’swimmers‘ itch'. It is suggested that the increased mortality arose through a combination of increased numbers of swans on the loch, and a fall in the water level of the loch which exposed the birds to previously inaccessible lead fishing weights and to the intermediate hosts of a range of internal parasites.