ACYTthosiphon kondoi Shinji is a major pest of alfalfa (Medicago sativa L.). Susceptible seedling plants are killed by A. kondoi, and established plants suffer yield loss by stunting. Information is meager on the influence of A. kondoi infestation or host plant resistance on forage quality of alfalfa leaves and stems. In a controlled environment, the forage quality (includingphenolic monomers)of alfalfagermplasmsKSI53P4and KS153BA3P4 (susceptible and resistant to A. kondoi, respectively) with and without A. kondoi infestation was compared. Leaves infested with A. kondoi had 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 with A. kondoi had higher neutral detergent fiber concentrations than did those of uninfested controls. Infested stems of KS153BA3P4had more syringic acid than did infested stems of KS153P4 and uninfested controls. However, A. kondoi infestation did not alter TIVDDM or digestible neutral detergent fiber of stems. The A. kondoi-resistant KS153BA3P4had 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 by A. kondoi decreased the nutritive value of leaves and stems.