Wolbachia (Rickettsiales: Anaplasmataceae) is a genus of cytoplasmically inherited alpha-proteobacterium that infects arthropods (especially insects) and filarial nematodes. In arthropods, upon infection, Wolbachia alters the host reproduction in a variety of ways including cytoplasmic incompatibility, parthenogenesis, male-killing and feminization to enhance its own transmission. My doctoral research has been focusing on molecular and biochemical interactions between Wolbachia and its arthropod host. In chapter one, I made an introduction of Wolbachia including its biology, phylogeny and potential in disease control. I also reviewed the studies on Wolbachia-host interactions with an emphasis on Wolbachia Type IV Secretion System (T4SS). In chapter two, I presented the research I carried out in order to understand Wolbachia-mosquito interactions through T4SS. I molecular cloned the T4SS coupling protein VirD4 from wPip and analyzed the co-transcription between T4SS and Wolbachia surface protein in wStr. In chapter three, I amplified and sequenced five ribosomal protein (rp) genes of wFol from its host Folsomia candida, and phylogenetic analyzed 17 Wolbachia strains based on the available rp sequences. I also detected a DNase activity in isolated F. candida DNA. In chapter four, I further studied the DNase activity that co-purified with F. candida DNA, including characterizing the DNase active conditions and estimating the molecular weight of the DNase. In chapter five, I described an approach to estimate the biomass of F. candida with a protein stain, Ponceau S.