Isopterygium tenerum (Sw.) Mitt., a species previously considered restricted to the New World, is reported for the first time from the Old World. Examination of the type collections of three South African species that were given names under Plagiothecium and Isopterygium, namely Plagiothecium rhynchostegioides Müll. Hal., P. sphagnadelphus Müll. Hal., and Isopterygium brachycarpum Dixon, revealed their conspecificity with I. tenerum. Accordingly, this species is established as one more Afro-American disjunct species. Some gametophytic structures of the type specimens of the South African species are illustrated.