Zampanolide (1), a 20-membered macrolide from a Tongan marine sponge, stabilizes microtubules and blocks cells in G(2)/M of the cell cycle. Zampanolide is cytotoxic in the low nanomolar range and induces microtubule bundles in cells. It leads to tubulin assembly in cells and in purified tubulin preparations and is not a substrate for the P-glycoprotein drug efflux pump. Zampanolide, with only four stereogenic centers, may be amenable to large-scale synthetic preparation.