Endosymbiotic bacteria are widespread in insects and mites. The maternally inherited endosymbionts can promote their spread and persistence in host populations by enhancing the production of females in infected hosts, either by improving overall host fitness or through reproductive manipulation. Cytoplasmic incompatibility (CI) is commonly observed and is expressed as a reduction in the number of offspring in crosses between infected males and uninfected females (or females infected with a different bacterial strain). CI is often related to the presence of Wolbachia sp. and Cardinium sp. Although, these two endosymbionts occur in arthropods and may infect the same host species, little is known about their interactions. Considering the importance and damage of the European red mite, Panonychus ulmi , in apple orchards, the aim of current study was to determine the prevalence and effects of Wolbachia sp. and Cardinium sp. in P. ulmi populations. Both symbionts were detected, using specific primers, in four tested P.ulmi populations. Four collected populations appeared to be infected with Cardinium sp. (10-100%) and infected with Wolbachia sp. (10-70%). Also, the tested populations appeared to be infected with both Wolbachia sp. and Cardinium sp. (10-60%). One of the population with dual infection (IWC) were treated by 0.1% (W/V) tetracyclin to remove Cardinium sp. infection and produce a population with the same genetic backgrand but infected by only Wolbachia sp. (Iw). Different crosses were performed between males and females of Iwc and Iw. No cytoplasmic incompatibility was found in Iw and Iwc crosses because of no decrease in the percentage of female progeny comparing to other crosses. Analysis of life table parameters showed that the intrinsic rate of increase ( r m ), finite rate of increase ( ? ), the age-specific survival rate ( l x ) and the age-specific fertility ( m x ) in IW population were significantly higher than those of Iwc population. The parameters of r m were determined 0.1 and 0.068 (1/day) and ? were determined 1.11 and 1.07 (1/day) in Iw and Iwc populations, respectively. However, the duration of first nymphal instar was longer in Iwc population than in Iw population. Other estimated parameters such as fertility, longevity and survival were not significantly different between Iw and Iwc populations. Therefore, based on the results of this study, there may be a complex interaction of costs and benefits among these two symbionts in dual infection. Understanding the influences of symbiotic bacteria such as Cardinium sp. and Wolbachia sp. in dual or single infections on the growth or reproduction of the Euroean red mite, can be used in the population dynamic studies or management programs. Key words: Cytoplasmic incompability, Life table, Fitness