In August 1970, a breeding aggregation comprising an estimated 13,000 Agalychnis spurrelli was observed at a man-made pond on the Osa Peninsula, Puntarenas Province. Costa Rica. Apparently this extraordinarily dense breeding population was the result of ideal conditions offered by the artificial breeding site: the lack of the more effective predators (fish) in a large permanent body of water surrounded by numerous suitable oviposition sites and continuous forest which is immediately accessible for post-metamorphic dispersal. A high degree of larval success followed by normal post-metamorphic mortality, unaffected by density dependent factors, could lead to the high breeding population observed. Some males at the site were seen to scrape previously laid eggs off leaves. Additional observations and experiments demonstrated a definite parachuting ability of A. spurrelli, a behavioral and morphologic adaptation shared to a greater or lesser degree with other highly arboreal tree frogs.