Four new species are described, viz.Ischnosiphon bahiensis(Brazil),I. ursinus(French Guiana and Rio Negro Basin),I. crassispicus(vicinity of Manaus), andI. fusiformis(NE Peru). New data on ecology, morphology, or distribution are presented forI. inflatusAnderss.,I. enigmaticusAnderss.,I. centricifoliusAnderss.,I. gracilis(Rudge) Koern.,I. helenaeAnderss.,I. macarenaeAnderss.,I. polyphyllus(P.&E.) Koern., andI. surumuensisLoes. The typification ofI. arouma(Aubl.) Koern. is briefly discussed.