Data on the yield of fisheries registered at the markets of Porto Velho, Guajará Mirim and Pimenteiras during the period 1980/84 are analysed. Also, the results of the taxonomic inventary of the ichthyofauna and the studies on the ecology of the principal species, which have been carried out in the basins of the rivers Jamari, Guaporé and Mamoré in the state of Rondônia, Brazil, through the POLONOROESTE Programme, are presented here.