Insulin-like growth factors (IGFs) play essential roles in fetal and postnatal growth and development of mammals. They are secreted by a wide variety of tissues, with the liver being the major source of circulating IGFs, and regulate cell growth, differentiation and survival. IGFs share some biological activities with insulin but are secreted in distinct physiological and developmental contexts, having specific functions. Although recent analyses of invertebrate genomes have revealed the presence of multiple insulin family peptide genes in each genome, little is known about functional diversification of the gene products. Here we show that a novel insulin family peptide of the silkmoth Bombyx mori, which was purified and sequenced from the hemolymph, is more like IGFs than like insulin, in contrast to bombyxins, which are previously identified insulin-like peptides in B. mori. Expression analysis reveals that this IGF-like peptide is predominantly produced by the fat body, a functional equivalent of the vertebrate liver and adipocytes, and is massively released during pupa–adult development. Studies using in vitro tissue culture systems show that secretion of the peptide is stimulated by ecdysteroid and that the secreted peptide promotes the growth of adult-specific tissues. These observations suggest that this peptide is a Bombyx counterpart of vertebrate IGFs and that functionally IGF-like peptides may be more ubiquitous in the animal kingdom than previously thought. Our results also suggest that the known effects of ecdysteroid on insect adult development may be in part mediated by IGF-like peptides.