The mipA gene of A. nidulans encodes a newly discovered member of the tubulin superfamily of proteins, gamma-tubulin. In A. nidulans, gamma-tubulin is essential for nuclear division and microtubule assembly and is associated with the spindle pole body, the fungal microtubule organizing center. By low stringency hybridizations we have cloned cDNAs from D. melanogaster and H. sapiens, the predicted products of which share more than 66% amino acid identity with A. nidulans gamma-tubulin. gamma-Tubulin-specific antibodies stained centrosomes of Drosophila, human, and mouse cell lines. Staining was most intense in prophase through metaphase when microtubule assembly from centrosomes was maximal. These results demonstrate that gamma-tubulin genes are present and expressed in humans and flies; they suggest that gamma-tubulin may be a universal component of microtubule organizing centers; and they are consistent with an earlier hypothesis that gamma-tubulin is a minus-end nucleator of microtubule assembly.