Rhizopus sexualis (Smith) comb. nov. is fully described morphologically. The most interesting points have proved to be (l) the production of zygospores ladderwise on one pair of zygophores; (2) that the regeneration of single suspensors has given only homo- thallic mycelium; (3) that both auxiliary ("Neben- sporangien") and normal sporangia are produced; (4) that plasma excretion occurs.The general cytology is described. No chromosomes are differentiated. No fusion of nuclei has been observed, even in zygospores several months old. Nuclei associate in pairs from the time of gametangium fusion, though it does not appear necessa.ry that each of a pair must come from a different gametangium. This association is still evident in the zygospores after several months, practically no nuclei being single. Some of the nuclei entering the zygospore degenerate. Germination of zygospores up to nineteen months old has not been obtained.Perfect hybrid zygospores have been obtained with Rhizopus nigricans (+)ve and (-)ve, mucor hiemalis (-)ve and Absidia cylindrospora (-)ve, and imperfect sexual reactions with a nunber of other species.Rhizopus sexualis appears to be of a predominantly (+)ve tendency; only one other such homothallic is known.Burgeff’s theory of changing sex is shown not to be consistent with events in Rhizopus sexualis. The conclusion is drawn, that the zygophores and progametangia are bisexual, though the sexuality of the gametangia has not been determined.It is finally suggested that fusion and reduction of the paired nuclei may take place on germination, though the possibility is envisaged, that the nuclei never fuse, and that there is only an association of nuclei
