A blue fluorescent compound was isolated from Morpho butterfly wings. Based on thin layer chromatographic, UV and CD-spectrophotometric and HPLC analyses, the blue fluorescent compound was identified as L-erythro biopterin. Biopterin is a major component of blue fluorescent pteridines in both M. sulkowskyi and M. adonis. Pterin and isoxanthopterin can also be detected as minor components in these species. This paper is the first to report the presence of biopterin in butterfly wings.