Mite identification is usually done by the examination of morphological characteristics, the variation of which is not always sufficiently known to allow reliable distinction between closely related species. Morphological variations can be associated with geographical distribution and/ or with host ranges. The objective of this study is to evaluate the morphological variations of a population of a predatory mite identified as Euseius concordis (Chant) (Phytoseiidae) collected in Piracicaba, southeastern Brazil, when reared at different combinations of temperatures (20°C or 28°C) and diets (pollen of Typha domingensis Pers or the eriophyid mite Aceria litchii (Keifer)). High variations were observed even among mites of each treatment. Temperature (especially) and diet caused significant changes in the dimensions of some structures. Extreme cases corresponded to the maximal lengths of the dorsal idiosomal setae z2 and z4, which were about twice their minimal lengths. It is argued that for practical taxonomic purposes, it is more relevant to determine the range of the variation of each structure than its mean or other statistical parameters describing populational variations.