Asteraceae species diversity is high in the xerophytic scrub of the Ecological Reserve of the Pedregal of San Angel (REPSA), located in the southern part of the Basin of the Valley of Mexico. Here we determined whether the frequency of polyploidy is high in the reserve, given the enhanced ability of polyploids to colonize new habitats. In addition, we compared the frequency of polyploidy in Asteraceae in the reserve with the frequency in three oceanic archipelagos and two continental areas in Mexico. This was done to see how the ‘virtual’ island of the open lava flow in the reserve compares with volcanic islands at different distances from source areas. Chromosome numbers for 75 species of Asteraceae were obtained from published literature. Based on the possession of three or more basic chromosome sets in a nucleus, 33% were polyploids. If taxa with haploid chromosome numbers of n ≥ 14 or n ≥ 11 were considered to be polyploids, the proportion of polyploids rose to 57 and 75%, respectively. When using a phylogenetic approach, the highest percentage of polyploids (84%) was obtained and it could be inferred whether they are palaeo- or neopolyploids; thus, we consider that this criterion better reflects the events of polyploidy in Asteraceae. A high frequency of polyploid species in Asteraceae in REPSA suggests that polyploids may have contributed to the species diversity and the vegetation structure of the xerophytic scrub of this reserve. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173, 211–229.