The present study was carried out to evaluate the possible allelopathic effects of Cynodon dactylon, Cyperus rotundus and Sorghum halepense on seed germination of Oryza sativa cv. IAC-165. Extracts from roots of Cynodon dactylon and Sorghum halepense and from tubers of Cyperus rotundas were used to evaluate the effects on rice seed germination. Aqueous solution of these organs, obtained by homogenization, filtration and centrifugation, were introduced on Petri dishes containing rice seeds. The evaluation of germination and initial growth showed a strong negative allelopathic effect of all three weeds studied.