SUMMARYOf 10 isolates ofCeratocystis ulmicollected in September, 1970, from eight trees ofUlmusXhollandicacl.‘Belgica’and two trees ofU.Xhollandicacl.‘Commelin,’seven were of compatibility type B and three, type A. Peeled, split, autoclaved elm twigs, dipped for 10 sec into a 3-da-old liquid shake-culture (Tchernoff medium) of a B isolate and, 3 da later, into a similar culture of an A, were compared in December, 1970, with similar twigs dipped first into A and 3 da later into B. Abundant, fertile perithecia formed in all but seven of the 42 possible combinations: an A isolate from Doorwerth, Netherlands, formed perithecia with each of the seven B isolatesonlyif it was thesecondinoculum. It thus acted as a male: able to serve as effector but not as receptor. These tests were made with 3-mo-old isolates. Reisolation from the same branch of the original tree in April, 1971, again yielded type A, but hermaphroditic rather than male. Passage of the male culture through elms yielded symptoms typical of Dutch elm disease but did not change its unisexual nature. Of two aggressive cultures received from England (both compatibility type B), one reacted as hermaphrodite and the other as male in more limited tests.