Wolbachia is a group of obligate symbiotic bacteria found in many insects and other arthropods. The presence of Wolbachia alters reproduction in the host, but the mechanisms are unknown. Molecular biological studies of Wolbachia have delayed significantly, and one of the reasons is the lack of transformation techniques of this bacterium. In the present study, bacteriophage particles were isolated from Wolbachia for the first time. The purified phage had an isometric head that was approximately 40 nm, in diameter and contained linear double-stranded DNA of approximately 20kbp. Partial sequence information (total of 20,484bp) revealed that there were 24 open reading frames including a structural gene module, and genes for replication and lysogenic conversion. This bacteriophage is the only known mobile genetic element potentially used for transformation of Wolbachia. (C) 2004 Elsevier Inc. All rights reserved.