The Indo-Pacific lionfish Pterois volitans is an invasive species that was first recorded in the Guanahacabibes National Park (GNP), a marine protected area in western Cuba, in 2009. In order to determine the invasion progression of this species, we studied lionfish abundance, size, and diet at 6 sites in the GNP between 2010 and 2014. The species' density, biomass, and length increased over this period, probably due to the abundance of food and shelter in the GNP. Analysis of stomach contents indicated that lionfish fed primarily on fish and crustaceans; main prey were teleosts, predominantly Gobiidae, Pomacentridae, Mullidae, Labridae, Scaridae, and Grammatidae. This example of a rapid increase in an unmanaged population at the onset of invasion provides information that can be used to design a management program targeting lionfish.