Elevation has strong effects on climate and biota. In Hawaii, elevation on volcanic mountains affects temperature, rainfall, vegetation, and fauna. The effect of elevation on animal distribution and abundance can be direct through effects on climate or, in the case of phytophagous insects, indirectly mediated through climatic effects on host plants. In montane wet forests dominated by Metrosideros polymorpha (ohia lehua) and Acacia koa (koa), longhorned wood-boring beetles (Cerambycidae, Plagithmysus) play an important role as decomposers of wood and as food for higher trophic levels. The beetles I studied feed exclusively on dead wood of koa. I assessed the effect of elevation on the density of longhorned beetles in plantations of koa at Hakalau Forest National Wildlife Refuge, Hawaii. Density per branch of longhorned beetles was more than twice as great in the low-elevation sample (1,620–1,650 m; 3.49 beetles/branch, n  =  200) as in the high-elevation sample (1,880–1,970 m; 1.61 beetles/branch, n  =  200; t  =  5.08, P < 0.001). Beetles were contagiously distributed among branches at both low and high elevations (CD  =  5.71). Size of branch had a consistent positive effect on density of beetles, although this effect was more pronounced at low elevation. Elevation affects biological diversity directly through climate and seasonality, and indirectly through climatic effects on key habitat features. Effects of seasonality and habitat features on longhorned beetles need further study in Hawaiian montane forest.