Horses are major contemporary dispersers of guanacaste tree seeds (Enterolobium cyclocarpum) in Costa Rica, and probably were in prehistoric times as well. By placing 2-L and 8-L piles of fresh horse dung containing 5, 125, or 500 guahacaste seeds each in grassland and adjacent deciduous forest (Santa Rosa National Park, Costa Rica) 1 determined that (1) the seeds have a much greater chance of being harvested by seed predator rodents (Liomys salvini) from the dung in forest than that in adjacent grassland, (2) an 8-L seed-rich dung pile hides a larger absolute number of seeds from rodents than does a 2-L seed-rich dung pile, (3) a seed has a much greater chance of being harvested from a seed-rich dung pile than from a seed-poor dung pile, and (4) the grassland rodent Sigmodon hispidus harvests sorne of the germinating guanacaste seeds from the dung but leaves hard dormant seeds behind. These findings suggest that a guanacaste seed dispersal agent that defecates small numbers of seeds in many small piles of dung in grassland will be a better dispersal agent for guanacaste tree seeds than one that defecates many seeds in a few large dung piles in the nearby forest.