The photochemical reaction dynamics of YcgF, a BLUF protein, were investigated by the pulsed laser-induced transient grating (TG) technique. The TG signal showed three reaction time constants: 2.7 mu s, 13 mu s, and 2 ms. The fastest was tentatively attributed to relaxation of the excited triplet state of the chromophore, flavin adenine dinucleotide (FAD), and the others represented conformational changes of the protein. The TG signal provided clear evidence that the diffusion coefficient (D) of the photoproduct (3.8 x 10(-11) m(2) s(-1)) was significantly less than that of the reactant (8.3 x 10(-11) m(2) s(-1)), with a time constant of 2 ms at a protein concentration of 700 mu M. Interestingly, the rate constant increased in proportion to the concentration of the protein, indicating that protein dimerization was one of the main reactions occurring after photoexcitation. The significant reduction in D indicates that a conformational change leading to an increase in interactions with water molecules occurs upon formation of the signaling state. The 13 mu s dynamics was attributed to the conformational change that induced transient dimerization. This conformational change might be an essential process for the creation of the signaling state. A detailed scheme for the photochemical reaction of YcgF is proposed.