Numerous dinosaur bone beds have been found in the Upper Cretaceous Two Medicine Formation. Taphonomy and size-frequency profiles for the most common skeletal elements of six predominantly hadrosaurid and (or) lambeosaurid assemblages were compared. All six localities are single, highly concentrated bone horizons within silty mudstones. Most are monospecific with some degree of skeletal association, parautochthonous, and likely the result of mass mortality. Four are associated with small lacustrine or waterhole environments and may be the result of drought-related mortality. Within the Two Medicine Formation drought was apparently a common mechanism for generating fossil assemblages.Both hadrosaurids and lambeosaurids appear to have been gregarious. Lack of association between small (total length < 3 m) and larger individuals suggests either that juvenile growth rates were as rapid as large ungulates or less likely, that juveniles had a period of noninteraction with adults, perhaps dwelling in strictly juvenile groups. In Maiasaura peeblesorum, reproduction appears to have been synchronous among herd members and seasonal. Additionally, mortality was high for juveniles and dropped off rapidly as animals approached adult size.