SUMMARY. The purpose of this experiment was to characterize a lesion of the rhamphotheca associated with tryptophan (TRP) deficiency, search for other histological abnormalities, and determine whether bird size and housing conditions are contributing factors to these lesions. Day-old broiler chicks (Ross × Ross 308) were placed in either floor pens with fresh pine shavings or Petersime battery brooders with two pens of 10 chicks each per treatment. Broiler chicks from 0 to 21 days of age were fed adequate (0.24%) and deficient (0.09%) levels of TRP in diets based on corn, corn gluten meal, and gelatin. Separate groups of control chicks were pair fed daily with the deficient chicks. Deficient chicks grew less efficiently than did the pair-fed controls. Upon gross examination, a lesion of the maxillary rhamphotheca in the vicinity of the nares was observed in 61% of TRP-deficient birds housed in the battery and 13% of the birds housed in floor pens. A similar gross lesion was only observed in one control bird. These lesions were located along the upper portion of the beak between the nares and appeared as a crusty or scab-like area on gross examination, composed of detritus, heterophils, and plasma protein. Inflammation occasionally was observed at the dermoepidermal junction. The incidence of lesions was reduced in floor pens compared to battery brooders, but similarly sized birds did not exhibit the lesion. The number of lesions seen grossly and histologically in TRP-deficient birds, as compared to control birds, supports the hypothesis that TRP deficiency is the primary cause of these lesions around the nares of broilers. Secondary environmental factors, perhaps coprophagy, also influence the incidence of the lesion.