The population structure of the twospotted spider mite, Tetranychus urticae Koch, and its sibling species the carmine spider mite, Tetranychus cinnabarinus (Boisduval) (Acari: Tetranychidae), from China were investigated using microsatellite markers. The two species exhibited significant polymorphism but generally low genetic diversity. The genetic differentiation between populations of T. urticae and T. cinnabarinus was extreme and was significantly related to geographical distance between the populations (Mantel test). The overall level of genetic diversity of T. cinnabarinus was significantly higher than that of T. urticae. However, the various geographical levels of T. cinnabarinus were lower than those of T. urticae. The average pairwise F_ST between geographical populations of T. cinnabarinus was far higher than that of T. urticae, suggesting that the genetic differentiation of T. cinnabarinus was much higher than that of T. urticae. A comparison of T. urticae and T. cinnabarinus collected in adjacent and sympatric areas provided no evidence that genetic differentiation was related to geographical distances, suggesting a lack of gene flow between populations of these two species. In a phylogenetic analysis, seven geographical populations of T. urticae clustered in one branch and six geographical populations of T. cinnabarinus were distributed in other branches.