Cytogenetic analysis using C-banding, silver staining and fluorescent staining was carried out on a population sample of Poecilia Iatipinna derived from Tampico, Mexico, to verify the presence of sex chromosomes in individuals from the southern areas of this species range and to investigate the extent of C-band and Ag-NOR polymorphisms. Females were found to have W heteromorphic chromosomes, with large amounts of heterochromatin-rich in AT nucleotide sequences. C-banding corresponded to the pattern proposed as typical for the genus. Specimens share one of the Ag- NOR locations previously described in populations from the U.S.A. and show additional ones as well.