Insulin-like growth factor I (IGF-I) is a 70 amino acid, mitogenic polypeptide, which, in mammals, acts through an endocrine, paracrine, and/or autocrine pathway to regulate growth and development. The primary goal of this study was to determine whether or not IGF-I-like immunoreactivity is present in the oviduct of the vitellogenic American alligator, Alligator mississippiensis, and if immunoreactivity patterns vary among the three functional oviducal regions: the albumen-secreting tube region, the anterior, fiber-secreting uterus, and the posterior, calcium-secreting uterus. Immunolocalization of IGF-I-like immunoreactivity was accomplished using a polyclonal antihuman rabbit antiserum with an immunoperoxidase staining system. IGF-I-like immunoreactivity was detected in all three oviducal regions of the vitellogenic alligator. The presence of IGF-I-like immunoreactivity in the oviduct suggests this hormone could function in the growth and proliferation of the alligator oviduct. Furthermore, the presence of IGF-I-like immunoreactivity in the tubal glands, which secrete components of the egg white, suggests that growth factors such as IGF-I may be synthesized by these glands and incorporated into the albumen during egg formation.