Polyclonal antiserum against Manduca sexta allatotropin has been used to investigate localization of allatotropin-immunoreactive cells in the larval brain, frontal ganglion and gut of the silk moth Bombyx mori. Manduca sexta allatotropin-immunoreactive (Mas-AT-IR) cells were found in the larval brains, but not in the prepupal brain. First instar larva has a brain with four pairs of bilateral Mas-AT-IR neurons, whereas second, third, fourth and fifth instar larval brains contained six pairs, five pairs, five pairs and ten pairs of labeled neurons, respectively. There were two large and two small neurons in the frontal ganglia of the larvae. Mas-AT-IR axons and cells were also found in the larval midguts.