Methodologies of competitive interaction quantification between crops are not widely investigated. Therefore, field experiments (using addition series) were conducted in 2005 and 2006, to quantify interspecific and intraspecific competition coefficients and, also, the relative competitive ability (RC) of potato ( Solanum tuberosum) and common bean ( Phaseolus vulgaris). The relations between potato tuber yield/plant and common bean density at various densities of potato and also with potato density at different densities of common bean are well described by the reciprocal equations. Potato tuber yield/unit area decreased as common bean density increased. Optimum potato densities on the basis of maximum potato tuber yield/unit area at 0, 20, 30, and 40 plants/m 2 of common bean were obtained at 14, 13, 12, and 12 plants/m 2, as estimated by the parabolic relations between potato tuber yield/unit area and potato densities at different common bean densities. Potato was a stronger competitor than common bean, as a potato plant was equal to 6.22 common bean plants, based on potato tuber yield. A common bean plant, also, was equivalent to 0.0475 of a potato plant, on the basis of common bean grain yield. Therefore, potato was more aggressive than common bean, indicating that potato tuber yield was mostly affected by intraspecific competition, whereas common bean grain yield was mostly affected by interspecific competition. Niche differentiation index (NDI) was smaller than 1, showing severe competition of two species for environmental resources. It was concluded that yield-density relations in intercropping could be well quantified by application of the proposed equations.
