Crop residues effect on the germination of rice (Oryza sativa L.) and its major weeds. Three simultaneous experiments were conducted in El Pelón de la Bajura Farm, Guanacaste, Costa Rica in 2003 to determine the degree of interference caused by varying amounts of previous rice crop (Oryza sativa L.) residue on the emergence of rice and associated weeds. Nine coverage treatments of CR1113 crop residues (0 to 9 t/ha) were applied to measure effects on rice and weed germination. The associated weeds included several of the most frequent in flooded rice: Oryza sativa, Echinochloa colona, Cyperus spp., Fimbristylis spp., Ludwigia spp. and Heteranthera limosa. In the first two experiments, inverse relationships were established between the degree of coverage and the density of emergent weed seedlings (R2>0.89) and the degree of coverage and the percentage of rice germination (R2=0.86), respectively. Using bean residue (Phaseolus vulgaris) in the third experiment but no suppressing effect on the germination of rice was observed, possibly due to its rapid decomposition. Although the germination of rice was not affected even at the highest level of residues, similar long-term trials should be conducted to evaluate the possible impact of residue decomposition, allelopathy and herbicide carry-over effects after various consecutive rice production cycles.