Crystals of biotite and phlogopite were deformed in simple shear to determine the influence of compositional and structural variables on behavior during mechanical deformation. The use of an interferometric strain/displacement gage (ISDG) with modified sample grips allowed the testing of small samples that did not contain significant partings along cleavage planes; these grips also permitted the use of very small compressive forces resolved normal to the basal planes. Overall differences in strength between samples from different localities could not be simply related to compositional differences (such as F content) or with the density of growth spirals perpendicular to the basal planes (which might impede the motion of basal dislocations). Differences in strength among samples from the same series were related to the orientation of the applied shear stress, and these strength differences correlated with F content. These represent the first known experiments with ISDGs in mineralogy and demonstrate that there is significant potential for their application to geological problems. ¿ 1999 American Geophysical Union