The Rb-Sr isotopic data for mineral separates from a rock of the Manicouagan impact melt sheet define an isochron of age T=214¿5(2&sgr;) m.y. and an initial Sr87/Sr86 ratio I=0.70991¿0.00007 (2&sgr;). This age is in excellent agreement with the K-Ar age of 210¿8(2&sgr;) m.y. of Wolfe <1971> and represents the time of crystallization of the melt sheet. Four whole rock samples yield age-corrected I values ranging from 0.70965 to 0.71006. These high I values are consistent with geological and geochemical arguments that the melt sheet was derived by total melting of Grenville basement rocks during the impact event 214 m.y. ago.