Approximately 2.5 million monarch butterflies in an overwintering colony in Mexico were killed by a severe winter storm in Jan. 1981. Flight testing of butterflies sampled from 4 locations within the colony during and after the storm gave an overview of the general flux of butterflies from clusters to the ground and back due to the storm''s effects. Normally adaptative behaviors to avoid freezing ground temperatures, including high roosting and crawling up onto low vegetation, were only partially effective against the severe cold. Selective wetting and subsequent freezing appears to account for the mortality patterns observed in the roosting clusters.