A block of fine-grained anorthosite consisting of 90 to 95 percent plagioclase and 5 to 10 percent mafics was collected from the AuSable Forks quadrangle in the northeastern Adirondacks. Its strength was determined in triaxial compressive tests for temperatures from 400° C to 1000° C and for confining pressures between 4.4 kb and 14.8 kb. The plagioclase has an average composition of An50 and an intermediate structural state. The strength of the anorthosite increases with increasing confining pressure and with decreasing temperature, ranging from 25 kb at 400° C, and 14.8kb to less than 2 kb at 1000° C, and 4.4 kb for 8 percent strain. Only albite and pericline twins were observed in the plagioclase before and after experimental deformation; failure under the conditions reported here was largely by shear fracture and cataclasis.