Abstract: Chemical analyses and structural formulae of biotites from the Zaër pluton, a zoned granitic body in the Central Hercynian Massif of Morocco, are presented. The pluton grades from more mafic single-mica granodioritic facies near the margin to more felsic two-mica granitic facies in the central part. There is a relationship between the composition of the biotites and that of the host rocks. Biotite composition in the internal two-mica facies was influenced by a hydrothermal phase.