In vivo, cytoplasmic microtubules are nucleated and anchored by their minus ends at the centrosome and are believed to turn over by a mechanism termed dynamic instability : depolymerization and repolymerization at their plus ends. In cytoplasmic fragments of fish melanophores, microtubules were shown to detach from their nucleation site and depolymerize from their minus ends. Free microtubules moved toward the periphery by treadmilling-growth at one end and shortening from the opposite end. Frequent release from nucleation sites may be a general properly of centrosomes and permit a minus-end mechanism of microtubule turnover and treadmilling.