Passive integrated transponder (PIT) tags have been used to mark a variety of organisms and have potential for marking poultry chicks. We examined the effects of PIT tags subcutaneously implanted in 3- and 7-d-old Leghorn chicks and found no significant differences over 40 d in survival or rate of daily body mass gain among tagged chicks and controls. The PIT-tagged birds were not more susceptible to pecking by other chicks than controls. No birds died, but 1 of 20 chicks lost its tag during the study. We believe that PIT tags provide a viable technique for marking individual juvenile birds, if tag loss can be reduced. Costs may be prohibitive in studies involving large numbers of birds.