In spermatids of Culex tigripes (Culicidae) the manchette is composed of numerous expansion-bearing microtubules which surround the nucleus. Under the action of podophyllotoxin, in zones where manchette microtubules are absent, the nuclear envelope bulges and forms abnormal extranuclear vesicles. We hypothesize that this newly recognized capacity of podophyllotoxin is due to its action on microtubule-membrane relationships.