EFFECT OF DIFFERENT SOYBEAN VARIETIES ON BIONOMICS OF TETRANYCHUS URTICAE (ACARI: TETRANYCHIDAE)Abstract : Population density and spatial distribution pattern of Tetranychus urticae Koch on seven soybean varieties including: Williams, Tellar, Zane, Sahar, Dpx, L17, Sari and one genotype, Ks3494, were determined in Tehran region, during 2007. The mean population density of the T. urticae per leaf on L17 variety (14.15) was significantly more than other varieties and the genotype of soybean. The lower density of the mite population was observed on Zane and Ks3494 (6.1 and 4.65 mite per leaf, respectively). Spatial distribution pattern of T. urticae was determined on these varieties of soybean using Index of dispersion, Regression models (Taylor and Iwao), Morisita's coefficient and Lloyd's mean crowding. The index of dispersion, Morisita's index and Lloyd's mean crowding indicated an aggregated pattern for spatial distribution of this mite in all soybean varieties and the genotype. The spatial distribution pattern of this pest using Taylor's power law and Iwao's patchiness in most cases was aggregated and in few cases random. It could be concluded that different varieties of soybean can affect bionomics of T. urticae. Spatial distribution parameter can be used to improve the sampling program and exact estimating the population density of this pest.