The small troops and roughly even adult sexual ratio of Alouatta seniculus, studied in Bush Bush Forest in Trinidad and on the ranch Hato Masaguaral in the llanos of Venezuela, are more similar to A. caraya than to A. palliata. Other aspects of the troop composition are discussed. Though the red howler prefers certain types of habitat including tall trees, it still occasionally must descend to the ground in the llanos. Range sizes are very variable though significantly less than those reported for A. palliata on Barro Colorado Island, but the red howler’s population densities in the study sites are at least equal to current estimates for Barro Colorado.