The isotopic composition of strontium in the different parts of the differentiated Skaergaard intrusion has been determined. The average Sr87/Sr86 ratio for the basic rocks was found to be 0·7065±0·002. Higher values, between 0·7101 and 0·7303, were restricted to the late-stage acid granophyres. The Sr87/Sr86 ratios for the basic Skaergaard rocks are similar to those found by previous workers. The enrichment in Sr87 expressed in the Sr87/Sr86 ratio is taken to indicate contamination of the acid granophyres by a source enriched in Sr87. From considerations based upon circumstantial evidence the average country rock, composed of old Precambrian acid to intermediate gneiss, is not sufficiently enriched in.Sr87 to account for the Sr87 enrichment observed in the acid granophyres by a simple assimilation process. At the present stage of the investigation the enrichment of the acid granophyres in Sr87 is unexplained.