Variation in mitochondrial DNA (mtDNA) was examined among 707 red snapperLutjanus campechanusrepresenting 16 samples taken during 3 years from localities in the northern and western Gulf of Mexico. Ninety-two composite mtDNA haplotypes were revealed by 13 restriction enzymes (representing 93 inferred restriction sites). Significant heterogeneity (P= 0.042) in mtDNA haplotype frequencies was detected among the 16 samples; however, homogeneity tests of mtDNA haplotype frequencies between or among samples taken in different years at the same locality and among samples at different localities within the same year, were not significant. No phylogeographic structure of haplotypes was evident, nor were rare haplotypes clustered geographically. Spatial autocorrelations did not differ significantly from those expected when no correlation exists. These findings are consistent with the hypothesis that a single breeding population of red snapper inhabits the northern Gulf of Mexico. Intrapopulational mtDNA diversities, however, differed significantly among samples, suggesting that red snapper in the Gulf may not be drawn from a single population. Red snapper in the Gulf of Mexico may possibly include recently derived populations for which there has been insufficient time for accumulation of significant differences in mtDNA haplotype frequencies.