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The riffle beetle genus Hexanchorus Sharp is, with 25 known species, the most speciose genus of the subfamily Larainae in the Neotropics and the second largest globally. An analysis of its phylogeny, based on two mitochondrial (COI, 16S) and two nuclear (18S, 28S) markers, including Hexanchorus-like, but morphologically distinct specimens, supported presence of an unknown genus. The new genus, Rumilaragen. nov., is described here with four new species (R. obscurasp. nov., R. paternasp. nov., R. riberaisp. nov., R. suppressasp. nov.) and their larvae. The separate position of the new genus is, beside molecular differences, well substantiated by the morphology of adults (the absence of sexual dimorphism, structure of the pronotum) and larvae (pleurites never reaching to sixth abdominal ventrite). H. sagittatusstat. nov. is elevated to species rank based on molecular data, and Rumilara leleupi (Delève) comb. nov. is transferred from the genus Hexanchorus and redescribed, based on its morphology.

The three genera and four species of Larainae (Elmidae) previously described from Australia are reviewed, and one new genus and seven new species are described: Australara glaisterigen. et sp. nov., Ovolara lawrenceisp. nov., Ovolara monteithisp. nov., Stetholus carinatussp. nov., Stetholus longipennissp. nov., Stetholus metatibialissp. nov., and Stetholus woronorasp. nov. A lectotype is designated for Hydora laticeps (Carter & Zeck), and the first new collection records of the species are reported since its description in 1932. The occurrence in Australia of Potamophilinus papuanus Satô, described from Papua New Guinea, is reported. A key to the species, photographic images of the external morphology and male genitalia, distribution maps, and habitat and behavioral information, when known, are provided for all twelve species of Australian Larainae.

The riffle beetle genus Hexanchorus Sharp, 1882 is distributed from Mexico to Argentina, forming an important component of the freshwater invertebrate fauna of Latin America. With 21 described species, Hexanchorus represents one of the most speciose Larainae genera, but its real diversity is likely much higher. We analysed material from a relatively small area in Ecuador, resulting in the first record of H.cordillierae for Ecuador and discovery of four new species and one subspecies: Hexanchorusvirilissp. n., Hexanchorusrostratussp. n., Hexanchorusshepardisp. n., Hexanchorusonoreisp. n. and Hexanchorusonoreisagittatusssp. n. For delimiting and characterizing species, both morphological and molecular (mtCOI DNA barcodes) data were used. A distribution map of Hexanchorus species is provided based on published records.