The Leiodidae (the round fungus beetles, the small carrion beetles, and the mammal nest beetles) of Atlantic Canada (New Brunswick, Newfoundland and Labrador, Nova Scotia, and Prince Edward Island) are surveyed. Twenty five species, including Colon (Colon) politum Peck and Stephan, Colon (Myloechus) forceps Hatch, Colon (Myloechus) incisum Peck and Stephan, Colon (Myloechus) schwarzi Hatch, Hydnobius arizonensis Horn, Anogdus dissimilis Blatchley, Anogdus potens (Brown), Cyrtusa subtestacea (Gyllenhal), Leiodes puncticollis (Thompson), Leiodes rufipes (Gebler), Agathidium atronitens Fall, Agathidium depressum Fall, Agathidium difforme (LeConte), Agathidium mollinum Fall, Agathidium oniscoides Palisot de Beauvois, Agathidium pulchrum LeConte, Agathidium repentinum Horn, Agathidium rusticum Fall, Gelae parile (Fall), Anisotoma blanchardi (Horn), Anisotoma discolor (Melsheimer), Anisotoma geminata (Horn), Anisotoma globososa Hatch, and Prionochaeta opaca (Say) are newly recorded in Atlantic Canada. One of these, Hydnobius arizonensis, is newly recorded in Canada. Colon (Myloechus) hubbardi Horn is newly recorded in the Maritime Provinces. Eight species are newly recorded in New Brunswick, 29 in Nova Scotia, two on Prince Edward Island, 12 on insular Newfoundland, and five in Labrador for a total of 56 new jurisdictional records. Catops paramericanus Peck and Cook and Catops simplex Say are newly recorded from mainland Nova Scotia, and records are provided to verify the occurrence of Leiodes impersonata Brown and Leiodes punctostriata Kirby in Nova Scotia, and Leptinillus validus (Horn) in insular Newfoundland. Three species, Agathidium hatchi Wheeler, Catops americanus Hatch, and Sciodrepoides watsoni (Spence), are removed from the faunal list of New Brunswick. As a result, 66 species of Leiodidae have now been recorded from Atlantic Canada. The name Anisotoma obsoleta (Horn) is revalidated while the name Anisotoma horni Wheeler is newly designated a synonym of A. obsoleta. The regional composition and zoogeography of the Leiodidae in Atlantic Canada are examined and species are grouped in six main categories, reflecting their distribution in the region. Island faunas are examined, particularly in regard to the similarities and differences of the faunas of Cape Breton Island, insular Newfoundland, and Prince Edward Island. Lone Shieling, in Cape Breton Highlands National Park, is highlighted as an apparent biodiversity 'hot spot' for this family within the region. Finally, the saproxylic component of the fauna (19 species in the Agathiidini) is briefly discussed, particularly in regard to potentially rare species, and the importance of monitoring populations of saproxylic beetles.