Trials were conducted to study the effect of experimental haemonchosis on dry matter intake, live mass gain, dry matter digestibility and .nitrogen retention in sheep. Six and 9 month old lambs were infested with a pure strain of infective larvae of Haemonchus contortus (3rd stage) under controlled conditions. Infestation resulted in significant decreases in nitrogen retention and dry matter digestibility (P < 0,01). These changes were mainly confined to the 2nd week and the latter stages of the 6 week experimental period. Decreased body mass gains by the infested lambs were less prominent; possibly being obscured by excessive fluid retention. Dry matter intake was not markedly influenced. These deleterious effects of haemonchosis are directly related to the severity of infestation.