Six fowl adenovirus strains belonging to four serotypes were administered orally and intranasally to 1-day-old chicks and aspects of their pathogenicity studied. Only one strain (HV 7) caused high mortality (approximately 30%), but body weight gain was significantly depressed 21 days after inoculation of four of the six strains. Although variations were found in the length of time that the viruses persisted in the tissues of infected chicks, one strain (KR 5) differed markedly from the remainder in that it was difficult to recover it from most tissues and because it failed to induce a serological response.