or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. known worldwide (Holm et al., 1977).This species has invaded agricultural fields in numerous countries and has reduced growth and yields of crops such as soybean [Glycine max (L.) Merr.], cotton (Gossypium hirsutum L.), peanut (Arachis hypogaea L.), upland rice (Oryza sativa L.), sugarcane (Saccharum spp.), and maize (Zea mays L.) (Holm et al., 1977;Strahan et al., 2000a). Rottboellia causes yield losses of 20 to 70%, depending on the crop type, the harvest cycle, and the local ecological conditions, including soil type, fertilization methods, and the season in which the crop is grown (Millhollon and Burner, 1993). In Mexico, Rottboellia has invaded fields cultivated with maize, rice, sugarcane, bean, and citrus, in addition to tropical forest plantations, such as mahogany plantations, along the coast of the Gulf of Mexico and the Pacific Ocean (Esqueda-Esquivel, 2005). Strahan et al. (2000b) showed that this invader weed could reduce maize yields by 33%. Other authors reported losses as high as 90% and proposed that the deleterious effect of Rottboellia on maize growth could be due to competition and/or allelopathy (Fisher et al., 1987;Delgado et al., 2006).Plants compete with one another for resources, such as nutrients, water, light, and pollinators. Some plants also produce and release chemical substances that inhibit or stimulate germination, growth, development, and survival of neighboring AbStrAct A greenhouse experiment was performed to test whether competition, and possibly allelopathy, were involved in Rottboellia cochinchinensis-maize interactions in two soil conditions: fertilized and unfertilized. Changes in morphological and physiological growth parameters for each species were determined by classical plant growth analysis. The dry weight and total leaf area of maize grown with Rottboellia decreased significantly as the density of the weed or maize plants increased, particularly under unfertilized conditions. At high densities of Rottboellia, regardless of fertilization, its root biomass allocation was 38% greater than the stem and leaf biomass allocation. In contrast, in unfertilized conditions, Rottboellia biomass allocation was higher in leaves than in roots and stems at high densities of maize or Rottboellia. Relative crowding coefficient values indicated that maize had a competitive advantage over Rottboellia. Mutual allelopathic effects of maize-Rottboellia interactions were not fully demonstrated because plant relative yield values were not significant among treatments. When the density of Rottboellia increased, its total leaf area and total dry weight significantly decreased, greater than when the density of maize increased. These results indicate that intra-specific competition between Rottboellia plants was greater than inter-specific competition with maize. High densities of maize significantly reduced the total leaf area of maize and Rottboe...