Scarcity of organic matter and nutrients in the topsoil is a typical feature of lowland primary tropical rain forests. However, clumped defecation by vertebrate herbivore troops and further dung beetle processing may contribute to locally improve soil biological activity and plant growth.We studied the impact of clumped defecation by the red howler monkey (Alouatta seniculus), a frugivorous primate, on the vertical distribution of topsoil (0-6 cm) main nutrients and microstructures in a tropical rain forest (French Guiana). Three latrines, where monkey troops regularly defecate, were sampled, together with adjoining controls for carbon, nitrogen, phosphorus and microscopic components.The vertical distribution of C and N was affected by clumped defecation: nutrients were mostly restricted to the top 2 cm in control areas while latrines exhibited homogeneously distributed C and N, resulting in higher C and N content below 2 cm. No marked effect of defecation was registered on Olsen P. A small although significant increase in pH (0.1-0.3 pH units) and a marked increase in soil respiration (¥1.5-2.5) were registered in latrines. Soil microstructures were studied by the small-volume method. Variation according to depth, site and clumped defecation was analysed by Redundancy Analysis. The three latrines were characterized by an increase in root-penetrated mineral-organic assemblages, mainly composed of recent and old earthworm faeces. The local stimulation of plant roots, microbial and earthworm activity was prominent, together with an increase in soil fertility. Consequences for the regeneration of tropical rain forests in the Amazonian basin were discussed, in the light of existing knowledge.