The fluorescence transient of Chlorella pyrenoidosa, excited by saturating light absorbed mainly by system II, has a dip D between the peak I at 75 msec and the large peak P at 400 msec (the times depend on light intensity). This dip is observed in aerobic cells and in anaerobic cells where it is prominent. In anaerobic cells, the I-D decline is hastened almost equally by absorption of either 705 or 650 nm background light. In anaerobic cells, supplementary 700 and 710 nm light given during the transient slightly hastens and heightens P. Methyl viologen, an exogenous system I electron acceptor, eliminates P. Results suggest that system I action causes D, and that P is due to reduction of Q (fluorescence quencher) and intersystem intermediates caused by development of a block in oxidation of XH (X being the primary electron acceptor of light reaction I). Mathematical analysis suggests that if only two forms of Q participate beyond I, then system I action is required for D. If three forms participate, then the system Q --> QH --> Q' (see text) may explain D. The Malkin model (14), in its present form, does not allow D.