ABSTRACTThe paper examines the geomorphological and sedimentological characteristics of the Rio Toro Amarillo alluvial fan located in the wet-tropical environment of the Atlantic watershed of Costa Rica. Alluvial fan development was found to be dominated by short periods of high sediment discharge to the fan, separated by longer, inactive periods characterized by weathering erosion, and reworking of fan sediments. Volcanic activity during 1963–65, and the resulting increase in sediment yield in the Toro Amarillo, produced major changes in fan characteristics over a ten year period. These changes are discussed. The length of the active period varies according to the magnitude, frequency, and type of sediment-producing event. It was estimated that the length of the inactive period for the Toro Amarillo may range from 100 to 500 years.The alluvial fan sediments are dominated by gravel that have a mean diameter about 30 to 40 cm. The sediment facies from the fan are described and their major characteristics outlined. Some comparisons are made between certain characteristics of the Toro Amarillo fan and alluvial fans in other climatic regions.