This paper is divided into three principal sections. The first section deals with the biology of four species of the genus Leiobunum; these were the most abundant species found in a sustained study of the phalangids of a mesic deciduous woodland carried out in 1957 and 1958 at UMBS, and they yielded a considerable body of information about their life history and ecology. The second section presents the results of laboratory experiments to ascertain the limits of tolerance and optimal conditions for temperature and moisture, factors of obvious importance in the phalangid environment. The third section attempts to integrate field and laboratory observations in analyzing the habitats of most of the Michigan species of phalangids as they were recorded in the course of statewide collecting.