Abstract Microbial symbioses likely drive the evolution of diet within animals, yet these symbiotic relationships remain poorly understood for many organisms. The bacterial endosymbiont Enterococcus faecalis is found in the intestinal tract of the beetle Harpalus pensylvanicus (DeGeer) (Coleoptera: Carabidae) and is thought to contribute to the digestion of the insect's seed diet. We tested whether E. faecalis increases seed consumption by H. pensylvanicus. The feeding assay consisted of 4 dietary treatments fed: 1) antibiotics and E. faecalis; 2) antibiotics and no E. faecalis; 3) no antibiotics and E. faecalis; and 4) no antibiotics and no E. faecalis, in which seed consumption of the beetles was measured. Beetles administered antibiotics and then E. faecalis consumed greater weights of seeds and had both decreased efficiency of conversion of ingested material to biomass (E.C.I.) per beetle and decreased efficiency of conversion of digested material (E.C.D.) to biomass per beetle. These data provide fur...