The mountain pine beetle (Dendroctonus ponderosae Hopkins) is native to western North American and is considered the most destructive insect within the coniferous forest.  Recent experiments have demonstrated reproductive incompatibility between geographically distant populations, while rangewide genetic analyses have described a broad isolation-by-distance pattern encircling the Great Basin Desert.  We present patterns characterizing postmating reproductive incompatibility within the mountain pine beetle with respect to host use, geographic patterns, neutral gene flow patterns, and degree of isolation.  To examine these relationships, eight populations were selected from varied hosts along a genetically divergent gradient and crossed in a common garden experiment.  Any incompatibilities in the F₁ will be based on comparisons of pairing success, brood production and sex ratio.  In addition, any incompatibilities in the F₂ (specifically hybrid sterility) will be determined by egg hatch ratios in backcrosses to fully fertile parental lines. Understanding these mechanisms that aid in reproductive isolation and facilitate speciation is critical to our understanding of species formation. Furthermore, any evidence of fully reproductively isolated populations could challenge the single species status of the mountain pine beetle.