We conducted an inventory of the fern, lycophyte and non-palm monocotyledon groundherbs of terra firme riparian forests in the lower Tapajos River basin of the Brazilian Amazon. Eight 1.5 x 250 m plots, totaling 0.3 hectares, were surveyed along the watersheds of the Cupari and Curua-Una tributaries of the Tapajos River, Floresta Nacional do Tapajos, Para, Brazil. To characterize the ground-herb community, we calculated species richness, abundance and Fisher’s alpha for each plot. To analyze floristic dissimilarity, we calculated the Bray-Curtis distance. In total, we sampled 3,130 individuals, 58 species, 27 genera and 20 families of riparian groundherbs. Marantaceae (14 spp) was the richest family and Poaceae the most abundant Family (738 individuals). The fern Triplophyllum glabrum (Tectariaceae) was the most frequente species, observed in 87.5 % of plots. Plots that showed the highest values for richness and species diversity were located in the Cupari River basin. The ground-herb community composition observed in the riparian zone here resembles that of other non-riparian forested sites in the Amazon with the plant families Marantaceae, Pteridaceae and Poaceae generally being the most commonly represented in the Amazonian ground-herb stratum.
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