The striped mullet Mugil cephalus L. is a circumtropical species whose extreme conservative morphology stands in contrast with the degree of genetic differentiation at a global scale. One hundred and fourteen mitochondrial control region DNA sequences were analysed from four localities in the Gulf of Mexico (Texas, Louisiana, Mississippi, Florida), one on the U.S.A. east coast (N. Carolina), and one in Hawaii, giving very high levels of molecular diversity (h=1·0 every haplotype was unique in all samples, =1·1-2·0% in Gulf-Atlantic and =3·1% in Hawaii). With no genetic evidence of dramatic population expansions, mismatch distributions were still very different in each ocean. Stable population levels in Hawaii have fostered the generation and persistence of very high molecular diversity, but Gulf-Atlantic samples suggest a shorter span of population stability. Ninety-five per cent of the molecular variance was allocated between ocean basins and virtually none among Gulf-Atlantic samples. A neighbourjoining reconstruction revealed Atlantic-and Pacific-specific lineages separated by more than 24% uncorrected sequence divergence (d=0·49 Tamura-Nei Gamma-corrected). The lack of phylogeographic structure among Gulf-Atlantic samples corroborated the AMOVA results and supported the existence of a single population with high levels of gene flow along the Gulf of Mexico and north-west Atlantic coasts. The genetic differentiation between oceans points to the absence of gene flow and an accelerated rate of mitochondrial evolution in the genus Mugil. 2000 The Fisheries Society of the British Isles

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