We found the third M intermediate (M-slow2) with a lifetime as long as that of N on the bacteriorhodopsin (bR) photocycle in alkaline suspension. While the reported slow-decaying M, M-slow1, forms under intense pulsed or continuous (stationary) light irradiation, M-slow2 needs both lights. M-slow1 is a cooperative product (Tokaji et al.) with a quantum yield that depends on both light intensities, whereas M-slow2 is formed from the species accumulated by the continuous light. These results show that formation process of M-slow2 is different from that for M-slow1. (C) 2010 Elsevier B. V. All rights reserved.