Light-induced absorption changes in an oxygen-evolving photosystem II (PS II) preparation from the thermophilic cyanobacterium Synechococcus sp. were analyzed using continuous illumination which caused the reduction of both Q A (first stable quinone electron acceptor) and Q B (second quinone electron acceptor of photosystem II). In this photosystem II preparation in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) the amount of Q A was estimated to be 1 per 42 chlorophylls. In the absence of DCMU, plastoquinone (1.68 per Q A) was photoreduced to plastohydroquinone within a few seconds, indicating that Q B is reduced and protonated during this period. An electrochromic band shift centered around 685 nm was observed with and without DCMU. The extent of this band shift caused by Q B reduction per electron was about a third or half of that caused by Q A reduction. A significant amount of cytochrome b-559 (0.86 per Q A) was photoreduced. Only 60% of the photoreduction of cytochrome b-559 was inhibited by a DCMU concentration that inhibited electron transfer beyond Q B, indicating that the site of the reduction of cytochrome b-559 is located before the Q B site and possibly on the donor side of PS II.