How the cell cytoplasm is spatially organized is of fundamental interest. In ordinary animal cells the cytoplasm is organized by a radial array of microtubules, called an aster. Aster microtubules are nucleated by the centrosome and elongate to the periphery. We investigated how asters grow in an extremely large cell, the frog egg, using microscopy of an extract system. Asters were initially nucleated at centrosomes, but then additional microtubules nucleated far from the centrosome, apparently stimulated by preexisting microtubules. The resulting growth process allows asters to scale to the size of huge egg cells while maintaining a high density of microtubules at the periphery. Microtubule-stimulated microtubule nucleation might be a general principle for organizing large cells.