In the past it has been assumed that the parental basaltic magma in ophiolites is subalkaline tholeiitic because so many of their lavas are compositionaly similar to midocean- ridge basalts. However, compositions of ultramafic and gabbroic cumulates in ophiolites are characterized by low total iron and titanium contents. These rocks show only insignificant iron enrichment during the course of their crystallization and differentiation. Early-formed cumulus olivine has a high Mg: (Mg+ Fe) ratio and plagioclase is calcic. These features indicate that the cumulate rocks derive from an alkali-deficient, calcic, and highly magnesian parental magma of komatiitic affinity. Highly magnesian lavas are locally associated with tholeiites in the Troodos and Hatay ophiolites of Mesozoic age as well as in the Canadian Appalachian ophiolites of early Paleozoic age. These unusual lavas display a komatiitic affinity in that they are characterized by high MgO, Cr₂0₃ and NiO contents, a low FeO: MgO ratio (<1) and very low contents of Ti0₂ (<0.4%), K₂O and P₂0₅. They have a correspondingly high normative and modal content of pyroxene and chrome spinel. The lavas show well-developed quench textures and represent chilled fractions of highly magnesian melts. Their crystallization order mimics the crystallization sequence of associated cumulate rocks. We suggest that the occurrence of these highly magnesian lavas, either early or late in the volcanic sequence, is related to a cycle of growth and decline of the subjacent magma reservoir controlling the fractionation processes.