Knowledge of the composition of the lower continental crust is important for understanding the formation and evolution of the crust as a whole, and the petrogenesis of continental basalts. Here we present rhenium-osmium isotope data for two well characterized suites of lower-crustal xenoliths from North Queensland, Australia, which have average major- and trace-element compositions similar to estimates of the bulk lower continental crust,. Our data indicate that the lower crust has 1 to 2 times as much osmium, about half as much rhenium, and is less radiogenic than the upper continental crust. We interpret the rhenium-osmium isotope systematics to indicate that assimilation and fractional crystallization are important processes in the formation of the lower crust, and lead to dramatic changes in the osmium isotopic composition of basalts that pond and fractionate there. A consequence of this is that the rhenium-osmium isotopic system should not be relied on to yield accurate mantle extraction ages for continental rocks.