Fluorescent chromatophores with bluish white fluorescence were detected in the guppy Poecilia reticulata. These chromatophores were observable only by means of a fluorescence microscope, and could not be observed by a standard transmission light microscope or incident light microscope. Resembling other common chromatophores, these fluorescent chromatophores were dendrictic cells and similar to other chromatophores in the size, about 50-100 μm. Of the 17 strains examined, these fluorescent chromatophores were detected only in the R strain, and in all 12 R strain individuals examined, each presented fluorescent chromatophores. These differences in the occurence of fluorescent chromatophores among strains suggest that the formation of these chromatophores is the result of genetic polymorphism.