A nephropathogenic Massachusetts strain of infectious bronchitis virus (IBV) designated H13-IBV was isolated from the kidneys of commercial broilers. H13-IBV caused respiratory distress, depression, and diarrhea in specific-pathogen-free chickens. Gross renal lesions included pale coloration, swelling, and urate deposition. Histologic renal changes were interstitial mononuclear cell infiltration and degeneration and necrosis of tubular epithelial cells. Lesions in respiratory tissues included thickening and edema of the air sacs, congestion of the tracheal mucosa, and frothy serous exudate. Histologic tracheal lesions were deciliation, mucous gland distortion, inflammatory cell infiltration, and squamous metaplasia. Clinically, H13-IBV was highly pathogenic in birds infected at 1 day of age and mildly pathogenic in birds infected at 4 weeks of age. Kidney lesions were of marked severity only in birds infected at 1 day of age. Tracheal lesions were similar in severity in both age groups.