Preliminary investigation on the slime mold Physarum gyrosum Rost. indicates that the presence of light and depletion of nutrients is necessary for initiation of fruiting. This is similar to the condition found by previous workers for species with pigmented plas- modia. Since P. gyrosum has pigmented plasmodia only when grown in the light several unanswered questions are raised regarding the function of the pigment(s) in initiation of fruiting in P. gyrosum. Numerous workers have reported on the effect of various factors on fruiting, or sporulation, in the Myxomycetes. Martin (1940), Hawker (1952), and Alexopoulos (in press) reviewed these studies. Most of the definitive work was done on the genus Physarum, with emphasis on Physarum polycephalum. There have been no reports on Physarum gyrosum Rost. During my investigation of the life cycle of P. gyrosum, thousands of agnotobiotic cultures1 were maintained under variable conditions. Records on these cultures show that fruiting occurred only in cultures which were transferred to nonnutrient agar or to half-strength corn meal agar and were exposed to light. This suggests that nutrition and light might be important for initiation of fruiting in this species. A trial experiment to determine the effect of light on fruiting in P. gyrosum was carried out. Plasmodial transfers from a culture, grown in variable incandescent light on oat flake agar, were made to six Petri plates, each containing ca. 30 cc 2% agar. Three plates were placed in the dark and three in continuous light, all at 25 C ? 2%. After one month fruiting had occurred in the three plates exposed to light but not in the plates kept in the dark. This experiment was repeated with 30 subcultures (plasmodial masses transferred from an oat flake agar culture to Petri plates containing ca. 30 cc non- nutrient agar). Fifteen stubcultures were placed in the dark and fifteen were placed in continuous light.2 All were kept in the same incubator at 20 C. The "light" cultures were examined daily. After five days 13 of the "light" cultures had fruited and examination of the "dark" cultures was started. Every third day a container, with two or three "dark" cultures, was opened. The results are given in Table I. All of the "light" cultures fruited while none of the "dark" cultures fruited prior to exposure to light. Eleven of the 15 "dark"