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The isotopic compositions of strontium in 25 basalts and related volcanic rocks from both oceanic and continental localities have been determined. The isotopic abundance of Sr 87 , conveniently expressed as the Sr 87 /Sr 86 ratio, was found to vary from 0·7047 to 0·7101. The range of variation is outside experimental errors and is considered to be significant. No consistent difference in the value of the Sr 87 /Sr 86 ratio was observed for basalts from continental and oceanic localities. The average Sr 87 /Sr 86 ratio for eleven oceanic basalts was found to be 0·7072±0·0003, whereas fourteen continental basalts and related volcanic rocks averaged Sr 87 /Sr 86 =0·7082±0·0003. The errors are the standard deviations of the mean. The average Sr 87 /Sr 86 ratio for all 25 basalts is 0·7078 with limits of variation of +0·002 and −0·003. The concentrations of rubidium and strontium in a representative number of the basalts were determined by isotope dilution. Five olivine basalts averaged Rb=17±6 p.p.m., Sr=390±55 p.p.m., Rb/Sr=0·044±0·018; whereas six tholeiitic basalts were found to contain 30±7 p.p.m. Rb, 504±167 p.p.m. Sr, Rb/Sr=0·060±0·024. On the basis of the most reliable rubidium and strontium analyses of igneous and sedimentary rocks available in the literature the Rb/Sr ratio of the near-surface part of the continental crust is estimated to be 0·25. If its average age is approximately 2 billion years and its initial Sr 87 /Sr 86 ratio was 0·704, the present average value would be 0·725. This estimate is compatible with results obtained for two composites of Palaeozoic shale which averaged Sr 87 /Sr 86 =0·7215±0·001. The hypothesis is advanced that there is sufficient enrichment of Sr 87 in crustal materials so that the value of the Sr 87 /Sr 86 ratio of igneous rocks at the time of crystallization can be used as a criterion for the origin of the material. The initial Sr 87 /Sr 86 ratio of an igneous rock formed by assimilation, remelting, or granitization of old crustal material enriched in rubidium is expected to be measurably higher than that of igneous rocks which are products of fractional crystallization of basalt magma or were derived from undifferentiated basaltic material at depth in the crust. The initial Sr 87 /Sr 86 ratio, time of crystallization, and possible co-magmatic relationships of differentiated intrusive igneous rocks or series of lava flows of different compositions can be indicated by the convergence of the whole-rock strontium development lines. This method is illustrated by the intrusive alkaline rocks of the Monteregian hills, Quebec.