Summary: The field relationships, petrography and mineralogy, including probe analyses of olivines, pyroxenes, and plagioclases, of the Karroo dolerite dike swarm of southern Malawi, are described. Rock analyses are given and define a typical tholeiitic trend of iron enrichment. The most magnesian rocks are thought to represent fairly picritic liquids. A number of authors have shown that there are significant geochemical differences between the basaltic rocks of the northern and southern parts of the Karroo outcrop in southern Africa, notably for elements such as Al, P, K, Ti, and Ba. The Malawi data presented here, however, show trends that are the reverse of those found hitherto, the Malawi dolerites being closely comparable with the rocks of the central Karroo basin of South Africa and Basutoland. The striking regional variation in the geochemistry of the Karroo basaltic rocks over southern Africa is independent of the tectonics or position vis-à-vis the central Karroo basin, and so probably reflects mantle heterogeneity.