twenty-six soil profiles were characterized and classified as ultisols in the middle-upper watershed of pirrís river. these soils developed on pleistocene to oligocene undulated to strongly undulated mountainous landscapes, on: 1) marine sediments, 2) volcanic andesitic-basaltic materials and 3) colluvial materials, all of them covered by natural vegetation of wet to very wet tropical forest in an ustic soil moisture regime. the soils are deep, possess an argillic horizon and are of fine to very fine texture. soil reaction is acid to strongly acid and may possess a horizon high in manganese concretions associated to ancient water levels of the pirrís river watershed, on geologic materials high in manganese content. clay content, cec, particle density and bulk density of the soils vary with parent material, while base content and phosphorus fixation are not so related. according to soil taxonomy, 2 suborders were recognized: humults and ustults. five great groups: haplustults, paleustults, haplohumults, palehumults, rhodustults; and 9 subgroups: typic haplustults, aquic haplustults, typic paleustults, andic haplohumults, aquic haplohumults, aquandic haplohumults, ustic haplohumults, andic palehumults, typic rhodustults. the andic and aquandic subgroups exhibit, in superficial horizons, depositions of volcanic ash materials dated around 700 years of age. buried clayey subsoils of talamanca ultisols are much older than their over laying horizon, being dated as old as 9000 to 65 000 years of age.