The γ-tubulin ring complex (γTuRC) is a protein complex of relative molecular mass~ 2.2× 10 6 that nucleates microtubules at the centrosome. Here we use electron-microscopic tomography and metal shadowing to examine the structure of isolated Drosophila γTuRCs and the ends of microtubules nucleated by γTuRCs and by centrosomes. We show that the γTuRC is a lockwasher-like structure made up of repeating subunits, topped asymmetrically with a cap. A similar capped ring is also visible at one end of microtubules grown from isolated γTuRCs and from centrosomes. Antibodies against γ-tubulin label microtubule ends, but not walls, in centrosomes. These data are consistent with a template-mediated mechanism for microtubule nucleation by the γTuRC.