Electron microprobe analyses of natural hydrothermal tetrahedrites reveal that most specimens are heterogenous in composition. A simplified formula for natural tetrahedrite, showing the three common solid solutions, is (Cu, Ag) 10 (Fe, Zn) 2 (Sb, As) 4 S 13 . Variations in Ag and Sr are positively correlated within each individual sample. However, the mathematical slopes and intercepts of the linear correlations are different for different samples, and the bulk composition of the samples varies with position within the ore deposit. Nearly 5,000 new point analyses are presented for tetrahedrites from Coeur d'Alene, Idaho, and Orcopampa and Julcani, Peru, to document these observations. Heterogeneity within a single sample could arise when a particular depositional site receives more and less evolved fluids at different times.--Modified journal abstract.