Ultrafast fluorescence dynamics of FMN binding protein (FBP) from Desulfobivrio vulgaris, strain Miyaxaki F, were compared in solution and crystal phases. Fluorescence lifetimes of FBP were 167 fs (96%) and 1.5 ps (4%) in solution (tau(av) = 220 fs), and 730 fs (60%) and longer than 10 ps (40%) in crystals (tau(av) = 4.44 ps). The quenching of the fluorescence of flavin in the protein was considered to be due to photoinduced electron transfer (ET) from Trp or Tyr to the excited isoalloxazine (Iso) nearby. The average lifetime was 20 times longer in crystal vs in solution. Averaged distances between Iso and nearby Trp-32, Tyr-35, and Trp-106 were 8.42, 7.36, and 8.15 A in solution, respectively (obtained by NMR spectroscopy), and 7.05, 7.72, and 8.49 A in crystal, respectively (obtained by X-ray crystallography). The prolonged lifetime in crystal cannot be elucidated by the change in the distances between the states. It was suggested that the longer lifetime in crystal was ascribed to the absence of water molecules around FBP with rapid motional freedom, which may be the driving force for the ET in flavoproteins.