Lignocellulosic feedstocks have high energy content and have been co-digested with sewage or manure biosolids in Europe for many years. However, it is unclear whether the current U.S. anaerobic digesters are capable of co-digesting lignocellulosic feedstocks without experiencing operational problems. We evaluated co-digestion of switchgrass with sewage biosolids under laboratory conditions similar to common U.S. digesters. Results indicated that finely-ground or ensiled switchgrass could be readily co-digested with sewage biosolids under typical U.S. digester conditions. Concentration up to 4% solids (representing up to 47% of VS added) achieved good specific methane yields and up to 74% energy conversion efficiency while maintaining acceptable VS removal. No evidence of solids accumulation, mixing problems, or floating debris was noted. However, fine-grinding switchgrass is energy intensive and likely to be cost-prohibitive. Moreover, ensiling produced a wide array of particle sizes and the effects of ensiling could not be fully separated from effects due to smaller particle size. Coarsely ground switchgrass, however, did not digest well. It had a low specific methane yield and quickly led to digester operational problems, even at the 2% solids level. Further research is needed to identify pretreatment methods that are more practical than fine-grinding. Ensiling appears promising, and should be studied under full-scale ensiling and digestion conditions to assure that observed effects were not due to smaller particle sizes achieved under laboratory conditions. Other, low-cost pretreatment methods also deserve study as a means of allowing lignocellulosic feedstocks to be co-digested in current U.S. anaerobic digesters.