A wide range of rock types (abyssal tholeiite, Fe-Ti-rich basalt, andesite, and rhyodacite) were dredged from near 95¿ W and 85¿ W on the Galapagos spreading center. Computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization. However, th P2O5/K2O ratio averages 0.83 at 95¿W and 1.66 at 85¿W and implies distinct mantle source compositions for the two areas. These source regions also have different rare earth element (REE) abundance patterns, with EF = 0.67 at 95¿W and 0.46 at 85¿W. The sequence of fractional lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95¿W. The mantle source regions for these two areas are interpreted to be depleted in incompatible (and violatile?) elements, although the source region beneath 95¿W is less severely depleted in La and K. Incompatible trace element abundances in 26 samples are used to infer that the range of Fe-Ti-rich basalt from 85¿W represents 19 to 35% residual liquid following crystal fractionation of a mineral assemblage of plagioclase, clinopyroxene, and lesser olivine. The most highly differentiated samples have also had less than 1% titanomagnetite removed. Most samples from 85¿W can be related to a common parental magma that contained approximately 9 wt %FeO*, 1 wt % TiO2, and had an Mg number (Mg3 = 100 Mg/(Mg+Fe2+)) of about 65. Although the samples from 95¿W cannot all be derived from a common parental magma, the inferred parental magmas may have been derived by varying degrees of partial melting of a common source. The fractionation sequence consists of two parts: an initial iron enrichment trend followed by a silica enrichment trend. We interpret the trace element data to indicate that the most iron rich lavas represent about 32% residual liquid derived by crystal fractionation of plagioclase, clinopyroxene, and lesser olivine from a parental magma with an Mg number of about 66. The silic enrichment trend results from crystallization of titanomagnetite and some apatite. Fractionation of pigeonite, which is a minor phase in the major element models, cannot be distinguished from clinopyroxene fractionation by using trace elements.