Eighteen non-marine mammal species (including seven species of bats) were recorded from a total of 49 islands in the Wessel and English Company island chains off north-eastern Arnhem Land, Northern Territory. Most individual species were restricted to, or had higher incidence on, larger islands, and species richness as a whole increased as island size increased. The most notable exception was the semi-aquatic Hydromys chrysogaster, which occurred relatively equitably across island sizes; this species, two bat species and the macropod Petrogale brachyotis were recorded from islands smaller than 10 ha. However, the variation between islands in the number of native terrestrial mammal species was not best predicted by island size, but rather by a combination of sampling effort and altitude (which explained 64% of the deviance in species richness), or altitude and distance to larger land mass (explaining 63% of deviance). Richness-area patterns for individual islands in these chains were reasonably consistent with those of other islands sampled in northern Australia. However, the fauna of the Wessel and English Company groups as a whole was less rich than that of the Pellew and Kimberley islands, and individual islands appeared to have lower species richness than comparable mainland areas. Species that were notably absent or that were recorded from relatively few islands include large macropods, Tachyglossus aculeatus, Antechinus bellus, Phascogale tapoatafa, Sminthopsis spp., Mesembriomys gouldii, Rattus colletti, Leggadina lakedownensis and Pseudomys calabyi. Some of these species may be absent through lack of suitable habitat; others have presumably disappeared since isolation, possibly due to Aboriginal hunting. Richness at the quadrat (50 x 50 m) scale was generally very low. Habitat relationships are described for the 7 species recorded from more than 5 quadrats. At a quadrat-scale, the richness of native mammals was greater on islands larger than 1000 ha than on islands smaller than 1000 ha. Quadrat-scale species richness varied significantly among the islands sampled by the most quadrats (even when the comparison was restricted to either of the two most extensive vegetation types), but this variation was not closely related to either area or altitude. The two most frequently recorded species, the rodents Melomys burtoni and Zyzomys argurus, showed distinct habitat segregation on islands where both were present, but tended to expand their habitat range on islands where only one of the species occurred. The most notable conservation feature of the mammal fauna of the Wessel and English Company Islands is the occurrence of the golden bandicoot, Isoodon auratus, a vulnerable species apparently now extinct on the Northern Territory mainland. Four feral animal species (Rattus rattus, Canis familiaris, Bubalus bubalis and Capra hircus) were recorded from a total of 6 islands.