The lionfish (Pterois volitans, P. miles) is the first known species of marine fish to invade the Caribbean and Gulf of Mexico, and it is threatening the biodiversity of the region´s coral reefs. Its success as an invasive species is due to its high predation and fertility, fast growth and lack of predators. Its first recorded appearance in Mexico was in 2009. Twenty-two sites were monitored around the reef of Banco Chinchorro Biosphere Reserve (BCBR), to estimate their abundance, during 2013. Densities from 0 to 333 ind ha-1 (97.6 ± 140.2 ind ha-1) and biomasses from 0 to 58.7 kg ha-1 (18.2 ± 29.9 kg ha-1) were recorded, the highest so far in the Mexican Caribbean. In addition, two lionfish distribution zones were detected: leeward reef (LR) and windward reef (WR). LR was 4.6 and 3.9 times higher in density and biomass than WR, respectively. The sizes found in the monitoring ranged from 5 to 40 cm of total length. Finally, a gregarious behavior was observed in 47.5% of the recorded fish. Our results suggest that to prevent the development of large reservoirs of lionfishes in the BCBR, management and control actions in areas of high lionfish abundance should be prioritized.