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Ulnar dimensions were measured in 14 species of armadillos (Xenarthra: Dasypodidae). An index of fossorial ability (IFA) was constructed, relating the length of the olecranon process to the remaining length of the ulna. For comparative purposes, the same measurements were taken in 14 other species of mostly South American mammals belonging to 3 orders and 11 families. The fossorial habits of these mammals were classified into 3 categories: (1) species mostly cursorial and non-digging; (2) species that often dig, but to which digging plays no essential part in their alimentary strategy and are not burrowers; and (3) species that are burrowers. IFA means of the studied mammal orders were compared using one-way analysis of variance on log-transformed data. Bivariate size allometry between ulnar dimensions and body mass was assessed by fitting (least squares and geometric mean) linear regressions of log-transformed data. It is concluded that the IFA discriminates among the species according to their fossorial habits within orders, but it is not equally useful in distinguishing fossorial species between orders. In armadillos, the relationships between ulnar dimensions and body mass are isometrical. Finally, the IFA is independent of body size.