During a detailed petrologic study of the pelitic schists of N.W. Maine (for location see Fig. 1, Guidotti, 1963) it became apparent that the basal spacings of muscovite in sillimanite-bearing rocks fall into distinct ranges corresponding to the metamorphic grade, Guidotti (1963), (1965A) and Evans and Guidotti (1966). This note is primarily concerned with the data on basal spacings which indicates that muscovite is very sensitive to changes in metamorphic conditions. It is suggested that other workers might find this easily measured parameter useful in studying pelitic schists.