The larva of Mecistogaster linearis is described and illustrated from specimens collected within or near the Río Dantas Wildlife Refuge at the north-western border of the Barbilla National Park on the Costa Rican Caribbean slope. Characters of F-0 larvae permit easy separation from Megaloprepus caerulatus, a species coexisting with M. linearis. Diagnostic characters include overall colour, shape of head, prementum and caudal gills. Exuviae may be determined using shape of mandibles. Two types of branched setae are present on tibiae and tarsi. Most are 3-branched but on front tarsi they are instead feather-shaped. It is suggested that these setae are used for eye-cleaning. M. linearis was a relatively rare but regularly occurring species in the study area throughout the 3-year study period. © 2005 Taylor & Francis Group, LLC.