Perthosite, a monomineralic saturated syenite of antiperthite, has a widespread occurrence in the basement complex of southern Nyasaland and is Precambrian in age. The perthosites are found with banded granulites and amphibolites into which they are observed to merge. They are considered to represent metamorphosed syenites that are late-stage differentiates of a basaltic or andesitic magma intruded during the Precambrian. The perthitic norite of the Lirangwe complex, 15 miles northwest of Blantyre, is described and is thought to be an intrusion of syenogabbro that was subsequently metamorphosed. An alternate possibility is that the perthites were formed by replacement, such as has been postulated for the mesoperthites of the Oersdalen area in Norway. The similarity of the Precambrian complexes to those of post-Karroo age, such as the ring complexes of the Chilwa series, is noted.