The foraging behavior of three colonies each of two species of leaf-cutting ants (Atta colombica and Atta cephalotes) was studied for a year in the seasonal evergreen forest of Guanacaste Province, Costa Rica. Foraging was predominantly nocturnal during the season and diurnal during the season, probably induced by seasonal changes in rain patterns, insolation, and vegetative cover. Largest quantities of vegetation were harvested at the beginning of the and seasons, in correlation with peaks in the number of plant species producing new leaves or flowering. Colonies harvested new leaves or flowers (especially during the season) as they became available. Though large amounts of mature leaves were available to them throughout the year, during the season the ants cut the mature leaves of a limited number of host plant species. It is proposed that Atta ants select new leaves, flower parts, and mature leaves of certain plant species because of their suitability for the growth of their fungus gardens and therefore the colony. ALTHOUGH THE EFFECTS OF SEASONALITY on temperate-zone biological communities are well known, the consequences of seasonal weather patterns on plants and animals in the tropics have only begun to be documented. Unlike temperate areas, variation in temperature is minimal. Seasonality is primarily manifested by distribution patterns of rainfall, and is expressed in terms of dry and rainy seasons. So-called rain forests usually have short seasons which appear to require little adjustment by organisms. In other areas the season is severe and lasts up to six months. To survive, plants and animals have evolved life-histories which enable them to avoid or even take advantage of the season (Fleming, Hooper, and Wilson 1972; Janzen 1967, 1970, 1973; Janzen and Schoener 1968; Wolf 1970). The season is a particular challenge to colonies of leaf-cutting ants (species of Atta). Leaf-cutters feed upon a fungus which they culture in their nest using leaves and other vegetative matter which they cut as a substrate for the fungus. Since most trees drop their leaves during the season, and herbaceous plants become inactive or die, the amount of potential harvest decreases greatly. As humidity drops and direct insolation increases, not only do conditions for foraging become unfavorable, but it becomes difficult to maintain the proper microclimate within the nest for growth of the fungus. Finally, species of Atta in the Guanacaste Province of Costa Rica mature reproductives during the season which are very large and make substantial demands on the fungus gardens. Mating flights occur at the start of the season (Rockwood 1972, 1973). The season may not be the only difficult season for leaf-cutters. During periods of heavy or continuous rainfall, foraging may be impossible or very difficult for several days. A number of workers are lost during each rainstorm (Rockwood, unpublished field observations; Weber 1972), and whole colonies may be lost during floods (Rockwood 1973). Since previous studies on leaf-cutter foraging have usually been of short duration (Cherrett 1968; Lugo et al. 1973), the effects of seasonality have been only briefly discussed in the literature (Weber 1972; Moser 1963, 1967; Lewis et al. 1974a, 1974b). This paper presents the results of a year's study and examines the effects of seasonality on the foraging of two species of leaf-cutting ants (A. colomnbica Guer. and A. cephalotes L.) in the highly seasonal Guanacaste Province of Costa Rica.