Only a few studies have examined the presence of endosymbionts in spiders (Araneae). We used PCR methods to assay for the presence of Wolbachia, Cardinium and Spiroplasma in 1000 spider individuals from China (including 16 families and 37 species). Of the species, 41.4% (15/37) contained one or more endosymbionts, Nephila clavata tested positive for all three endosymbionts, indicating that hereditary symbioses are extremely widespread in spiders. Align ment of the sequences obtained here with previously published alignments allowed us to establish phylogenetic trees for three symbiotic bacteria. The results showed that the Wolbachia found in infected spiders all belong to group A or B. The maximum parsimony(MP) tree showed that all Cardinium and Spiroplasma infecting spiders formed the same branch, indicating a very close genetic relationship between the two genera of symbiotic bacteria that exist in the spiders from different countries. However, the sequences of Spiroplasma from Chinese spiders showed a closer phylogenetic relationship and greater similarity to each other.