The ichthyofauna of the headwaters of the main tributaries of the mighty Solimões/Amazonas River has been little studied. Considering the importance of those environments for the overall fish diversity in that river system, we surveyed the composition of the fish fauna of the upper portion of Purus River and two of its tributaries (Caeté and Macapá rivers), state of Acre, Brazil. The collections were done in November 2004, using a seine net and a set of gillnets of different mesh sizes. A total of 735 specimens belonging to 86 species and 28 families were collected. Eight species, Creagrutus occidaneus, Phenacogaster pectinatus, Prionobrama filigera, Moenkhausia cf. lepidura, Leptagoniates pi (Characidade), Henonemus punctatus (Trichomycteridae), Toracocharax stellatus (Gateropelecidae), and Eigenmannia macrops (Sternopygidae) composed nearly half of the specimens collected. This survey adds 48 new records to the ichthyofauna of Purus River and elevates to 243 the number of known fish species in that river, but a greater sampling effort is necessary to produce a reasonably complete picture of the fish diversity in the basin.