Philopotamidae is a large caddisfly family with 20 extant genera comprising about 1 000 described species worldwide. Alterosa contains 22 described species distributed in South and Southeastern Brazil. Adults of the genus are very similar in general structure, but are remarkable for their wide variation in male genitalic morphology. The specimens were collected with Pennsylvania light traps and preserved in 80% ethanol. Alterosa nessimiani sp. nov. is described and figured from male specimens collected in streams of Rio de Janeiro state, Southeastern Brazil. The new species is closely related to A. jordaensis Blahnik 2005, from which it can be distinguished by the presence of two spines on the intermediate appendages and by the shape of tergum X.
Keywords: words, alterosa nessimiani, atlantic forest, caddisfly, neotropical region, taxonomy