Morphological and zoogeographical data indicate that North American pygmy shrews are referable to the Holarctic genus Sorex and to the Nearctic subgenus Microsorex Coues, 1877. Microsorex has its closest affinities with the North American autochthonous subgenus Otisorex . These subgeneric relationships are represented in a phylogenetic classification. Univariate and multivariate analyses support the conclusion that the subgenus Microsorex is monotypic, containing the species Sorex hoyi . Five subspecies are recognized based on these analyses. Variation in the species is generally that of decreased overall size from north to south. In addition, there are several zones where the species exhibits a rapid change in size. These zones demarcate subspecies boundaries. There is no demonstrable secondary sexual variation and individual variation is slight. There are significant changes in the skull and in the head and body length with increased age. The roots of the first upper incisors grow continuously, thus farther separating the cingulum from the alveolus with increased age. The degree of separation between the cingulum and alveolus is used as the index to age. Wear on the first upper incisors is rapid. Their continued down and back growth is necessary for continued occlusion with the lower incisors.