2-Alkyl-benzimidazoles are inhibitors of photosynthesis in intact Anacystis cells, depending on the length of the alkyl side chain. The extent of inhibition increases with increasing chain length then falls again, the maximum being at eleven carbon atoms in the side chain. This inhibition of photosynthesis is due to an uncoupling of photophosphorylation as demonstrated in a cell-free system of Anacystis. Non-cyclic photophosphorylation coupled to ferricyanide reduction in this cell-free preparation is more sensitive to undecyl-benzimidazole than cyclic photophosphorylation.