Megabalanus peninsularis is a key species in the rocky shore, particularly in Malpelo Island in the Eastern Tropical Pacific (ETP) where it dominates the mesolittoral zone and the infralittoral fringe.The size structure of M. peninsularis was determined to infer the population state (pyramid model) by measuring the basal diameter of individuals (n= 837) around the oceanic island.The species population distribution followed a bimodal pattern (class I-II and VII).The pyramid expansive model, with a relatively higher number of juveniles than adults suggests that the population is growing.