Ecology. Amazon and Chiquitano lowland forest to Yungas and Tucuman-Bolivian montane forest; on soil, humus, leaf litter, logs and base of trees. Elevation. 190-2340 m. Distribution. Bolivia: Beni, Chuquisaca, Cochabamba, La Paz, Pando, Santa Cruz, Tarija. Widespread in the Neotropics, eastern North America.

      Isopterygium tenerum is characterized by the ovate-acuminate leaves (mostly less than 1.2 mm long), few quadrate to short rectangular alar cells, 2-4 along margin, and strongly constricted below the capsule mouth. This species is the most common member of the genus in the lowland Amazon and Chiquitano forest, and lower Yungas montane forest; generally replaced by Isopterygium byssobolax in Tucuman-Bolivian montane forest in southern Bolivia. Isopterygium tenerum is likely to be confused with I. byssobolax, the former is distinguished by the 1-3(4) quadrate cells along the leaf base margin, the latter by the more numerous (4-10) quadrate cells.Taxicaulis cylindraceus Müll. Hal., described from a collection made by Germain in 1889 in the vicinity of Choquecamata, Department of Cochabamba, is a synonym.