Vertical transmission of larvae is a major pathway in the life cycle of several species of Strongyloides, but evidence for it occurring in humans or dogs with Strongyloides stercoralis is absent. In an effort to determine if vertical transmission could occur with S. stercoralis, each of 3 female dogs was infected with filariform larvae at a different stage of the reproductive cycle, i.e., preconception, gestation, or postpartum. Results showed that none of 6 pups born to a female infected before conception or any of 6 pups born to another female infected during gestation harbored any stage of S. stercoralis when necropsied at parturition. Conversely, all 5 pups that nursed from the female infected immediately postpartum became infected with adult S. stercoralis in their small intestines (range, 56-129 adult worms). Significantly, live filariform larvae of S. stercoralis were observed on 2 different occasions from milk samples taken from the lactating female. Because arrested development of larvae is not known in S. stercoralis, there is no reservoir of larvae in the parenteral tissues of females to queue for passage to the pups and, thus, it is not surprising that only timely infections, perhaps very late in gestation and during lactation, can be successful. These data support previous work in dogs with S. stercoralis, which concluded that vertical transmission through prenatal pathways does not occur, but they are the first from the dog to indicate that vertical transmission of this parasite through transmammary routes is possible. Whether transmammary transmission of S. stercoralis occurs in humans remains unknown but given its immense pathological potential, it should not be overlooked.