Some midocean ridge basalts from the East Pacific Rise, the Galapagos Spreading Center, and the Juan de Fuca Ridge are characterized by high FeO*, TiO2, MnO, P2O5, Na2O, K2O, and SiO2 and by low MgO, CaO, and Al2O3 relative to normal midocean ridge basalt. Graphical data display and least squares petrologic computer mixing calculations based on major elements, and phosphorus fractionation calculations indicate that these basalts are generated by fractionation of plagioclase, clinopyroxene, nad minor olivine in the average proportions 9.3:7.7:1. The calculations are compatible with a model in which up to 74% of the parental magma has fractionally crystallized to generate the ferrobasalts. Apparent variations in the normative and major element compositions of midocean ridge basalts from fast and moderate spreading east Pacific ridges can be explained by variable degrees of shallow crystal fractionation rather than by different depths of magma generation as was previously suggested.