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The genus Percichthys (Serranidae) includes three nominal species in Argentina, trucha, vinciguerrae and altispinis. The authors of this paper examine materials from: 1: the Río Negro river in its inferior course, in front ofViedma; 2: lake Pellegrini, near Neuquén, where the rivers Neuquén and Limay meet and form the Negro; 3: Plottier, near the place just named; 4: Colorado river, in Fortín Uno; 5: Curacó river, a tributary to the Colorado, now cut into separate sections since years ago on account of the lack of water; this river normally would connect the Colorado with the rivers up to the San Juan where the « trucha » lives; 6: Luro or La Salada lagoon, formed by the Colorado river near its mouth; 7: Argentino lake, in the southern Patagonia.These fishes are known as « trucha criolla » or « native trout » although the old Spanish name was « perca », more appropiate. Percichthys altispinis Regan 1905 is a good species ; it has been re-found in the Colorado river, at Fortín Uno. An illustration of it is given, characters of four specimens and a note on its scales.P. trucha C. V. reveeals itself on close examination as a complex species or linnean species (linneon) ; withseveral combinations of characters, but even more materials are needed to establish if there are geographical races (subspecies).A new examination of the Chilean materials is required (former authors considered them jointly with the Atlantic versant or Argentine materials).Some of the infraspeciíic forms are prognathous, and low finned ; others, the contrary; the head may be normal, or conical and bony; etc. As to P. vinciguerrae its standing as a valid species is doubtfull; perhaps, with P. laevis Jenyns it is a southern form.In the same reduced habitat (lagoon, or isolated course) diversified forms are present; some show parallelism with those of other places ; it is supposed that they show ecological influences according to the year or season of birth or developpment.A thorough study of the scales is given, with epidological characteristics and general conciusions as to the method of measuring and comparing their « reading». There are some marked differences even in the same habitat. Instituto del Museo de la Unviersidad Nacional de La Plata Probiota : Programa para el estudio y uso sustentable de la biota austral - Serie Documentos Relacionados Debe citarse: MAC DONAGH, E. & A. THORMÄHLEN DE GIL. 1945. Observaciones sobre las especies de truchas criollas. Rev. Mus. La Plata, n. s., Zool., Argentina, 4(27): 139-193. Series: Zoologia no.27