A number of specimens of the shorttail shrew from northern Lower Michigan were collected from study plots which were under microclimatic investigation. These were examined for possible age determination criteria and it was found that tooth wear was the most reliable of any criteria examined. It was found that the Blarina of the region had a complete breeding season, the onset of which varied in different microhabitats. Some of the implications of this complexity are discussed. The Blarina population on an area of virgin northern hardwoods was found to have a sex ratio of 1 male to 2+ females, while the ratio on areas of secondary succession was found to be the reverse. The usual 2 male to 1 female ratio is explained on the basis of males being the primary invaders of areas of secondary succession.