The present paper deals with five species of Amaranthus, i. e., Amaranthus viridis L, Amaranthus deflexus L, Amaranthus spinosus Linn, Amaranthus retroflexus L, Amaranthus hybridus L, that occur in Campinas as weeds. Besides the description of the species, an analytical key for their determinations is also supplied. According to the results, the genus Amaranthus is preferentially disseminated in "terra-roxa" type soils. In dry sandy soils the genus does not occur, and in damp soils only the species Amaranthus deflexus L was detected.