" Blue spiny lobster Panulirus inflatus (Bouvier, 1895) is an economically and ecologically important species along the Pacific coast of Mexico. Because most studies have been done on the local level and conducted to obtain fishery biology data, little is known about population boundaries. Previous analysis showed slight morphometric differences between lobsters along the Baja California Peninsula coast and those from the mainland coast. To determine whether differences are consequences of local variation or reproductive isolation and genetic variations, the genetic structure of P. inflatus was determined using sequence data of mtDNA (control region, 12S gene, and 16S gene). Genetic variability (number of haplotypes, haplotype diversity, and nucleotide diversity) for each mtDNA fragment was similar among localities. Nonsignificant differences among localities were revealed (control region: Φst = –0.0027, p = 0.555; 12S gene: Φst = –0.0011, p = 0.466; 16S gene: Φst = –0.00834, p = 0.759). Low levels of differentiation in morphology and the absence of variability of genetic structure suggest that the blue spiny lobster represents a panmictic population with phenotypic plasticity. We suggest that the lack of variation in genetic structure is related to oceanographic processes coupled with an extended larval period. Mismatch analysis suggested that the population history is characterized by range expansion."