ALTHOUGH THE members of the Acrasiales have been known to exist for almost a century, only recently has there been an increasing and widespread use of the members of this order as experimental organisms, with great interest being shown in their behavior. It is therefore essential that their life cycle should be known and that there should be as little doubt as possible regarding their behavior. Since the work of Skupienski (1920), the only works, insofar as the writers know, on nuclear behavior in the Acrasiales have been made by Bonner and Frascella (1952) and Wilson (1952a, 1953). In a preliminary report Wilson (1952) confirmed Skupienski's (1920) account of sexuality in Dictyostelium and carried the cytological investigation further (Wilson, 1953) to describe meiotic divisions of nuclei at aggregation and mitotic divisions just prior to spore formation. Bonner and Frascella (1952) differed with the above findings and stated that they found no evidence of sexuality and only mitotic divisions at aggregation. Sussman (1955, 1956), using induced mutants, has failed to find recombination in cultures of mixed populations; and although the svidence is negative, Sussman regards it as proof that sexuality does not exist in the Acrasiales. Because of these differences of opinion, there has been a hesitancy on the part of other investigators to accept any of the above interpretations. MATERIALS AND METHODS.-In the present study the following isolates were used: five of Dictyostelium discoideum Raper, one from K. B. Raper, two induced mutants (NC-4,W and NC-4,B) from M. Sussman, one isolated at Mountain Lake, Va., and one isolated at Montreal; one of D. purpureum Olive isolated at Cambridge, Mass.; two of Polysphondylium violaceum Brefeld, one from K. B. Raper and one isolated in Montreal; one of P. pallidum Olive isolated in Montreal. All isolates were cultured with Escherichia coli on yeast-starch agar (Emerson, 1941) and maintained at room temperature (18-25?C.). The agar plates were inoculated with a suspension of bacteria and set aside until a good growth of bacteria had covered the plate. Spores from the Acrasiales were inoculated into the center of the plate so that subsequent growth was circular and the various stages of the life-cycle did not intermingle. Plates prepared in this manner last for several days, with a new crop of aggregations every day. The mutant strains of D. discoideum received from Dr. Sussman (NC-4,B and NC-4,W) were grown together by inoculating the prepared agar plates with spores