AbstractAnimals serve as hosts for complex communities of microorganisms, including endosymbionts that live inside their cells.Wolbachiabacteria are perhaps the most common endosymbionts, manipulating host reproduction to propagate. ManyWolbachiacause intense cytoplasmic incompatibility (CI) that promotes their spread to high and relatively stable frequencies.Wolbachiathat cause weak or no CI tend to persist at intermediate, often variable, frequencies.Wolbachiacould also contribute to host reproductive isolation (RI), although current support for such contributions is limited to a few systems. To test forWolbachiafrequency variation and effects on host RI, we sampled several localProsapia ignipectus(Fitch)(Hemiptera: Cercopidae) spittlebug populations in the northeastern USA over two years, including closely juxtaposed Maine populations with different monomorphic color forms, “black” and “lined”. We discovered a group-BWolbachia(wPig) infectingP. ignipectusthat diverged from group-AWolbachia—likemodelwMel andwRi strains inDrosophila—6to 46 MYA. Populations of the sister speciesProsapia bicincta(Say) from Hawaii and Florida are uninfected, suggesting thatP. ignipectusacquiredwPig after their initial divergence.wPig frequencies were generally high and variable among sites and between years. While phenotypingwPig effects on host reproduction is not currently feasible, thewPig genome contains three divergent sets of CI loci, consistent with highwPig frequencies. Finally, Maine monomorphic black and monomorphic lined populations ofP. ignipectusshare bothwPig and mtDNA haplotypes, implying no apparent effect ofwPig on the maintenance of this morphological contact zone. We hypothesizeP. ignipectusacquiredwPig horizontally as observed for manyDrosophilaspecies, and that significant CI and variable transmission produce high but variablewPig frequencies.