ABSTRACT: The aim in this work was to evaluate the coexistence of Urochloa decumbens (signal grass) and Synedrellopsis grisebachii (Cinderella weed) in a replacement series experiment. To find the critical population density, 1, 2, 4, 8, 16, 24 and 32 plants were planted in the same box, harvested after 40 days, dried and weighted, and analyzed through regression analysis. The replacement series experiment used density percentages for signal grass and Cinderella weed of 100:0, 75:25, 50:50,

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... 5 and 0:100. The aboveground parts were harvested, dried and analyzed through qualitative and quantitative measures. Sixteen plants per box were needed for the signal grass to achieve the critical population. Qualitative analysis showed mutual harm to the dry matter allocation of both species. Quantitative analysis showed Cinderella weed to be a better competitor than signal grass.