1. By use of flow cytometry we measured nuclear DNA content in cells from 16 stocks representing 9 species of the genus Xiphophorus. 2. Significant differences were detected between certain stocks and species with respect to DNA content. 3. Male-female differences were apparent in 5 of 7 stocks in which males and females were studied. 4. Estimation of nuclear DNA content is of potential significance in connection with the genetics of sex determination and the study of taxonomic relationships.