We report the finding of the first haploid-diploid-triploid mosaic fish from the family Poeciliidae. The animal was derived from a laboratory cross of a female F1 hybrid of Poecilia mexicana and P. latipinna with a male from an ornamental strain derived from P. mexicana and P. sphenops (Black molly). It was identified because of its unusual pigmentation pattern and molecular methods (flow cytometry, NOR staining) confirmed its mosaic genotype. The mode of mosaic formation and the possible importance for poeciliid fish evolution are discussed.