This paper evaluates the establishment of the wMelPop strain of Wolbachia in field sites in Australia and Vietnam. The authors describe an initial increase in Wolbachia frequency in all sites examined‚ followed by an eventual decline most likely due to deleterious fitness effects associated with this strain. The authors conclude that‚ for the wMelPop strain to successfully establish in wild mosquito populations‚ additional supplementary strategies would be required. In contrast‚ the wMel strain of Wolbachia has been shown to readily invade local areas under a range of conditions.