This invention relates to novel fluorescent GFPs and BFPs. A novel BFP according to this invention has an F64L mutation as well as a L236R mutation and is provided with improved fluorescence. Furthermore, another BFP has the F64L mutation with the characteristics as described above and other mutations, V163A and S175G, and it possesses markedly improved characteristics in the expression at 37.degree. C. in addition to those as described above.