Even though the insects of the Neotropical area had been neglected, several authors have studied different aspects of the Southern Neotropical Plecoptera (Aubert, Benedetto, Enderlein, Froehlich, Gerstaecker, Illies, Navás, Klapálek, Werner, Zwick), specially the Andean oligostenothermal cool-adapted stoneflies. Most of the Argentinian river basins are widely spread out. The upper reaches of the Negro hydrographic system are on the east slope of the Andes and discharge into the Limay and Neuquén rivers downstream. The Negro river is formed by the convergence of both rivers and presents a sequence of reservoirs (main-stream impoundments on Limay and off-channel ones at Neuquén) ; it reaches the South Atlantic coast at approximately 41 ° S. The headstreams of the Valdivia system are on the west slope of the Andes. This river, much shorter than the Negro, joins the Pacific at 40° S. Transition from rhithron to potamon is rapid, if indeed the potamon exists. Macrozoobenthic samples of these two North Patagonian river basins, Negro (Argentina) and Valdivia (Chile) were taken in the period 1977-1983 in order to compare habitats of aquatic insects and other invertebrates on both slopes of the Andes and at the same latitude. Ten genera of Plecoptera were recorded by the author as nymphs : Gripopterygidae (Antarctoperla, Limnoperla, Potamoperla, Pelurgoperla, and Araucanioperla ?), Diamphipnoidae (Diamphipnoa and Diamphipnopsis), Austroperlidae (Klapopteryx), Notonemouridae (Austronemoura) and Perlidae (Kempnyella). Most of these genera of Plecoptera were collected in the upper rhithral. Only Antarctoperla and Limnoperla were found in reservoirs, in one lake and in medium-size to wide rivers. Gripoperygids were more frequent on the Argentine side ; the other families were more abondant on the opposite slope ; this situation can probably be explained by the different ecological conditions brought about by cattle raising and dam construction.