Two new species of Helobata Bergroth, 1888 (Hydrophilidae: Acidocerinae) from Brazil are described and illustrated. Helobata amazonensis sp. nov. is described based on an adult male specimen from the Amazon River floodplains, Amazonas State. Helobata pantaneira sp. nov. is described based on an adult male specimen collected from swamps in the Pantanal biome, in Mato Grosso State. Helobata corumbaensis Fernández & Bachmann, 1987, H. larvalis (Horn, 1873) and H. quatipuru Fernández & Bachmann, 1987 are recorded for the first time from Mato Grosso; Amazonas, Ceará and Mato Grosso; and Rio de Janeiro States respectively. Diagnosis, illustrations, a checklist and a key to Brazilian species of Helobata are provided.

Four new species of Desmopachria from the Amazon river floodplain (várzea) are described and illustrated, D. manus sp. nov., D. varzeana sp. nov., D. cavia sp. nov. and D. nigrocapitata sp. nov., together with new distributional records of D. balionata Miller, D. draco Miller, D. subnotata Sharp and D. striola Zimmermann for Amazonas and Pará states in Brazil.

Two new species of Notomicrus Sharp, 1882 from Brazil are described and illustrated: N. meizon sp. nov. from the state of Amazonas and N. teramnus sp. nov. from the state of Minas Gerais. The new species can be distinguished from all other members of Notomicrus mainly by their aedeagal features, especially for N. teramnus sp. nov., with a unique aedeagal structure. In addition, new records from Brazil are provided for N. brevicornis Sharp, 1882 (states of Amazonas and Pará), N. gracilipes Sharp, 1882 (Amazonas and Rio de Janeiro), N. malkini Young, 1978 (Amazonas, Pará and Rio de Janeiro), and N. traili Sharp, 1882 (Amazonas, Goiás and Paraná).

Three new species of Bidessodes (Coleoptera, Dytiscidae) are described and illustrated from the Amazon river floodplain (“várzea”): B. nessimiani sp. nov., B. hamadae sp. nov., and B. demarcoi sp. nov. Bidessodes evanidus Young is recorded for the first time from Amazonas and Pará states, Brazil. All species are included in the subgenus Bidessodes s. str. An identification key of Bidessodes s. str. species, modified from Young (1986), is provided.

The genus Xenelmis is found in the Nearctic and Neotropical regions. Currently, seven out of thirteen species are registered from Brazil. Here, two new species of Xenelmis from Brazil are described and illustraded-Xenelmis sinecarina sp. nov. and Xenelmis granatoides sp. nov. Additionaly, X. granata and X. micros are characterized and illustrated.