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Ferocactus histrix is a barrel cactus that is widespread in Mexico. A population located in Llanos de Ojuelos, a semiarid zone representative of many disturbed regions in north-central Mexico, was studied. Over a period of 10 years (1997 to 2007), the average number of individuals decreased from 21.95 to 3.53 plants per 300 m(2). A change in population size structure was also registered over this period of time. In 2008, a plot selected on the basis of plant abundance was established within the population and a genetic analysis was conducted with ISTR and ISSR markers. This analysis revealed low levels of genetic diversity (expected heterozygosity (H-E) = 0.073, Shannon index (I) = 0.113 and H-E = 0.178, I = 0.271, respectively) compared with those of most studied cacti species. The genetic diversity between the different life stages was also evaluated, and a gradual decrease in levels of genetic variation was observed from adults to juveniles and seedlings (H-E = 0.130, I = 0.192 to H-E = 0.103, I = 0.157). These differences, however, were not significant. Loci fixation and a decrease in the frequency of rare alleles were observed in seedling and juvenile classes. The decline in genetic variation may be associated with recent bottlenecks experienced by the population of F. histrix. If the sizes of local populations of F. histrix continue to decrease, genetic variation will be gradually lost, and the risk of extinction will increase.