Using methods to evaluate ecological niche breadth and overlap, I studied the host-specificity and the host-selection of 11 species of gamasid mites collected from 25 species of small mammals in western Yunnan, China, Levins' niche breadth was used for host-specificity, while Colwell-Futuyma's niche overlap and a fuzzy clustering analysis were used for host-selection. Of the 11 species of gamasid mites, the host-specificities of Laelaps nuttalli and Laelaps echidninus are the lowest (wide niche breadth), and those of Laelaps algericus and Laelaps chini the highest (narrow niche breadth). Mites with little taxonomic affinity tend to parasitize the same hosts (high niche overlap). Taxonomically close mites, however, tend to choose different species of small mammal hosts (low niche overlap). The results indicate that Levins' niche breadth is an effective method for evaluating host-specificity of gamasid mites and other ectoparasites. That there is an inverse relationship between the closeness of gamasid mites in host-selection suggests that there might be an interspecific competitive mechanism in operation.