ABSTRACTThe development of the multinucleate condition ofPeronospora tabacinasporangia was studied at 15 and 23 C using the fluorescent stain mithramycin. Sporangiophores and sporangia from infected leaf disks were collected 7 h after induction of sporulation and thereafter at 30 min intervals for 5 h. Since the developmental age of sporangia varied among sporangiophores within samples, length of sporangia was used as a proxy class variable for time. Initially, one nucleus migrated from the sporangiophore into each small (3μm). spherical sporangium. A predominant number of the developing sporangia in the 5–10μm classes incubated at 15 C had either 2, 4 or 8 (2n) nuclei. Chi-square analyses of the number of sporangia in these nuclear classes supported the hypothesis that nuclei were derived from 1 (22) or 2 (23) synchronous divisions of the daughter nuclei from the initial nucleus and not from random divisions or nuclear migration from the sporaqgiophore. Nuclei in developing sporangia incubated at 23 C exhibited one synchronous division (22). Subsequent divisions occurred randomly. The average number of nuclei in mature sporangia of the 3 isolates examined was approximately 15.