The Sn content, determined by electron microprobe, of 36 biotites from granitic rocks of the Afu Complex ranges from 50 to 1,000 ppm and averages 500 and 150 ppm for biotites from mineralized and barren granites, respectively. These results show that the Sn content of biotites associated with mineralization is three times greater than the Sn content of biotites from barren granites. Thus, the possibility of differentiating between mineralized and unmineralized granite on the basis of the Sn content of biotite is suggested.The relationship of the tin content of the biotites to that of the rock is nearly constant at approximately 10:1. The biotites have less than 40 percent of the amount of tin than the granites have. The concentration of tin in the biotites appears to be a function of the chemical evolution of the granites. Sn may increase in the biotites with decreasing oxygen fugacity and temperature of crystallization. The low oxygen fugacity and low temperature of crystallization seem due largely to the high concentration of F, Rb, and other volatile elements in the magma.