Background. Biological invasions are often listed among the main threats to the ecosystem and are considered drivers of biodiversity loss. The Indo-Pacific lionfish, Pterois volitans (Linnaeus, 1758) (hereafter lionfish), invaded the Atlantic Ocean where it threatensthe stability of the marine ecosystem. It would be crucial to know its biological characteristics to understand how environmental parameters could affect its growth. It is also important to continue the earlier efforts of management and control. In this study, we described the L–W and L–L relations and the relative condition factor of lionfish in two natural protected areas in the southern coast of Quintana Roo, Mexico. Materials and methods. Lionfish were captured during 2012 and 2013 from the Reserva de la Biosfera Banco Chinchorro (RBBC) and the Parque Nacional Arrecifes de Xcalak (PNAX). The length–weight relation was calculated based on the equation  W = aTLb. The relative condition factor was calculated through the relative weight. Results. A total of 817 lionfish were caught in the frames of this study. In this number, there were 449 individuals from the RBBC (282.1 ± 62.1 mm TL) and 368 from the PNAX (249.2 ± 77.6 mm TL). The L–W relation for lionfish from the RBBC was W = 0.0041 TL3.258 and that for the PNAX was W = 0.0049 TL3.191. There was a significant difference between these relations (ANCOVA, F = 3.91; P = 0.0481). The growth type was positive allometric. The L–L relation was significant. The relative condition factor differed between areas only in 2013, but a high value was determined in 2012. Conclusion. The L–W  relations were different between locations (RBBC and PNAX) but no between years. The relative condition factor showed high values (>100) for both locations which may imply that lionfish is in good shape, in the studied location, due to environmental factors providing good food supply and because of the lack of predators. These results may be useful as a baseline to document the population dynamics of lionfish in the region.

L–W relation, relative condition factor, Yucatan Peninsula, Banco Chinchorro, Xcalak