The grey mullet, Mugil cephalus, plays an important role in the fisheries and aquaculture of tropical and subtropical regions of the world. This species is considered cosmopolitan, but its distribution appears peculiar with regard to the coastal ecology of the species. A multidisciplinary study of the geographic variability of this species, through a cytogenetic, molecular and morphometric characterization, was undertaken to detect whether genetically distinct populations occur. The preliminary results from analyses of mitochondrial DNA and of chromosomes of seven different populations around the world are reported. The different populations analysed are well discriminated by mtDNA analyses: samples are clustered in four groups, Mediterranean, East Atlantic, Central Pacific and East Pacific, with a maximum sequence divergence of 3.3%. The karyotype of all the populations studied is uniformly composed of 48 acrocentric chromosomes, and neither heterochromatin distribution nor nucleolus organizer regions allow the identification of chromosomal markers useful in distinguishing these genetically differentiated groups of populations. © 1993.