The present study was carried out to evaluate the possible allelopathic efecfts of Cyperus rotundus L., Sorghum halepense L., Cynodon dactylon L., Canavalia ensiformis D.C., and Brassica napus L. on seed germination of Lycopersicon esculentum Mill. cv. Santa Cruz. Extracts from tubers of C. rotundus, roots of S. halepense and C. dactylon, and leaves and roots of C. ensiformis and B. napus were used to evaluate the effects on tomato seed germination. Aqueous solution of these organs, obtained by homogenization, filtration and centrifugation, were introduced on Petri dishes containing tomato seeds. The evaluation of germination showed a strong negative allelopathic effect of C. rotundus tubers, S. halepense and C. ensifor mis roots on tomato seeds. Extracts from leaves of B. napus and C. dactylon roots inhibited germination of 'Santa Cruz' tomato seed also.