The symbiotic relationship between microorganisms and arthropods is very common in nature. Some of the symbiotic microorganisms alter the normal sex-determining mechanisms in arthropods, causing sex-ratio deviation, killing males, and eventually leading to asexual reproduction and the possibility of species extinction. In the biocontrol of agricultural pests, predatory mites of the family Phytoseiidae are of great importance. Here, regarding the observance of sex-ratio deviation in Amblyseius swirskii (Acari: Phytoseiidae), the presence of three reproductive manipulator bacteria, i.e. Wolbachia, Rickettsia, and Cardinium were investigated in adult mites. Firstly, investigation of the female: male ratio in commercial mites showed that the ratio was about 42:58. Thus, there was an obvious deviation from the normal sex ratio (50:50). Therefore, to determine the presence of sex-determining bacteria, the whole DNA was extracted from A. swirski mites, or alternatively the adult mites were used. Using specific primers of the above-mentioned bacteria for WSP, gtlA, and 16S rDNA genes, multiplex PCR was applied. The PCR products were sequenced, and the sequences were processed using MEGA 6 software, blasted, and checked within the NCBI gene bank. The results indicated the simultaneous presence of Wolbachia sp. and Rickettsia sp. in A. swirskii in all individuals tested, which meant 100% colonization. Therefore, the sex-ratio deviation in this species could be explained by the presence of sex-ratio distorting symbionts, although more studies are needed to prove this concept. Moreover, using individual adult mites directly in PCR tubes as the sources of raw DNA saved time and reduced the costs and labors in detecting the bacteria. Multiplex PCR also decreased the need for reagents, enzymes, DNA templates, etc., and saved a significant amount of time.