Although tomato psyllid annually causes significant losses in potato and tomato crops in Eastern Mexico and the Central United States, infestations in Western North America have been historically rare. However, substantial populations appeared in 2001 in Western North America and caused losses in tomato production exceeding 80%; losses in 2004 reached 50%. To determine if these new outbreaks were the result of a simple range expansion or the evolution of a new B. cockerelli biotype, the ISSR markers, COI , ITS2 and wsp sequence data were used to characterize populations of the psyllid. Western populations from Baja, Orange Co. and Ventura Co. were compared with populations from the Central USA (Colorado and Nebraska) and Eastern Mexico (Coahuila, Mexico). Based on the ISSR markers, the psyllid populations clustered into two groups, with one group including populations from Western North America and the other group including populations from the Central USA and Eastern Mexico. For COI comparisons, there was one base pair difference found in the 544 bp-long COI fragments, but the populations again segregated along the same geographic lines. Two strains of Wolbachia were identified, the maximal differences between wsp clones from all populations was 5 bp for strain Bac1 and 23 bp for strain Bac2 out of a 555 bp fragment. The ISSR data therefore were consistent in indicating development of a new psyllid population that has adapted to Western North America, but the other genetic data sets were less conclusive.