The rocks of Tahiti constitute what is probably the most complete suite of strongly alkaline rocks in the Pacific Ocean. New analytical and petrographic data show that the differentiated rocks form two divergent series. Each series is represented by a wide range of effusive and plutonic rocks that differ mainly in their relative degree of silica enrichment. The end members of one series are trachytes and syenites that are almost exactly saturated; the second series trends toward phonolites and nepheline syenites. Fractionation of the principal minerals of the plutonic suite (feldspars, silica-poor pyroxenes, kaersutite, biotite, and iron-titanium oxides) should have caused strong enrichment of silica in residual liquids and could not have been the sole controlling factor in differentiation. The composition of crystalline phases seems to have been a response to magmatic differentiation rather than its cause. The compositions of the differentiated liquids can be explained as the products of differentiation in a vertically spreading zone of fusion in the upper mantle.