Five flavonoid compounds, including two new isoflavanones, were isolated from the root bark of Erythrina x bidwilli. Their structures were determined to be 2, 10-di(γ, γ-dimethylallyl)-3, 9-dihydroxypterocarpan (erythrabyssin II), 6, 8-di(γ, γ-dimethylallyl)-7, 2', 4'-trihydroxyisoflavanone (bidwillon A), 8-γ, γ-dimethylallyl-2', 4'-dihydroxy-[6'', 6''-dimethylpyrano-(2'', 3'' : 7, 6)]isoflavanone (bidwillon B), 8-γ, γ-dimethylallyl-7, 4'-dihydroxyisoflavone (8-γ, γ-dimethylallyldaidzein), and 8-γ, γ-dimethylallyl-5, 2', 4'-trihydroxy-[6'', 6''-dimethylpyrano-(2'', 3'' : 7, 6)]isoflavone (auriculatin), by means of spectroscopic analysis. Some potent activities against oral microbial organisms (Fusobacterium nucleatum and Prevotella intermedia) were shown in these flavonoid compounds.