AbstractDrosophila Kc cells exposed to physiological doses of the moulting hormone, β‐ecdysone, elongate, become motile, and subsequently aggregate. This pattern of morphogenesis was found to require the assembly of a microtubular cytoskeleton. Tubulin content was significantly increased in hormone‐treated cells when compared to controls, as measured by a3H‐colchicine‐binding assay. However, determinations of rates of tubulin synthesis and breakdown revealed no difference between control and hormone‐treated cells for either parameter. When tubulin content was assayed by methods that do not depend on colchicine‐binding activity, no difference between hormone‐treated and control cells was observed. These results are discussed in terms of a model in which β‐ecdysone affects the distribution of tubulin in “assembly‐active” and