The problems produced by invading species have been recognized in the last years, considering the situation the second cause of biodiversity lost, after habitat degradation. The lion fish (Pterois volitans), a species native to the Western Pacific, was first detected in southern Florida in 1985. Since 2000 the species began dispersing, extending first to the east coast of the United States, Bermuda, and Bahamas, and, after 2007, it has expanded to the Caribbean, appearing in Colombian oceanic and continental localities in 2008 and 2009, respectively. San Andrés natural conditions ease deeper understanding of several aspects of the invasion and constitutes a natural experiment to study the process and behavior of the colonization. Estimations of its abundance, density and size structure in the island environment of the Colombian Caribbean should allow making valid approximations to mitigating strategies. 1200 m2 distributed in three stations (Bajo Bonito, Villa Erika, and West Point) were studied in the island. A density of 379.03 ind/ha (± 220) and a total population of 109.6826 x 104 individuals with a mean size of 29.03 (± 2.47) cm were found