Acyrthosiphon kondoiShinji is a major pest of alfalfa (Medicago sativaL.). Susceptible seedling plants are killed byA. kondoi, and established plants suffer yield loss by stunting. Information is meager on the influence ofA. kondoiinfestation or host plant resistance on forage quality of alfalfa leaves and stems. In a controlled environment, the forage quality (including phenolic monomers)of alfalfa germplasms KS153P4 and KS153BA3P4 (susceptible and resistant toA. kondoi, respectively) with and withoutA. kondoiinfestation was compared. Leaves infested withA. kondoihad lower concentrations of crude protein and true in vitro digestible dry matter (TIVDDM) than uninfested leaves of controls, but infested stems had higher crude protein concentration. Stems and leaves infested withA. kondoihad higher neutral detergent fiber concentrations than did those of uninfested controls. Infested stems of KS153BA3P4 had more syringic acid than did infested stems of KS153P4 and uninfested controls. However,A. kondoiinfestation did not alter TIVDDM or digestible neutral detergent fiber of stems. TheA. kondoi-resistant KS153BA3P4 had more neutral detergent fiber in leaves but lower TIVDDM and digestible neutral detergent fiber in stems than susceptible KS153P4. Infestation of alfalfa for short time periods byA. kondoidecreased the nutritive value of leaves and stems.