The glass-frogs (Family Centrolenidae) of Costa Rica comprise 13 species all placed in the genus Centrolenella. A review of ihe fleischmanni group indicates thai six species: C. fleischmanni, C. colymbiphyllum, C. chirripoi, C. valerioi, C. talamancae, and C. vireovittata, a new species, from southwest Pacific Costa Rica, occur in the republic. Salient features for distinguishing the species include head structures and proportions, degree of tympanum development, finger webbing, color patterns and male calls. The nominal taxa C. chrysops Cope, C. decorata Taylor, and C. millepunctata Taylor, are placed in the synonymy of fleischmanni; C. reticulata Taylor is regarded as a synonym of C. valerioi. Detailed distributional analysis for all 13 Costa Rican species of the genus indicates that most forms occur in lowland or premontane slope evergreen forest. As many as six or seven species may occur at the same locality, although few species show consistent co-occurrence at many sites. No obvious ecologic factor explains the diversity and differences in species composition from site to site. The mosaic distributional pattern and a similar unique mosaic of basic and derived features distinguishing each species makes determination of relationships within species groups difficult. Within the fleischmanni line, chirripoi, talamancae, and fleischmanni appear closely allied. Centrolenella vireovittata resembles this stock in basic coloration and male call but is unique in the family in having a striped dorsum. Centrolenella valerioi and C. colymbiphyllum do not appear to be closely related lo any other forms, although the two may be included with vireovittata as a subgroup based upon characteristics of the head and snout region.