Apolipoprotein D (ApoD) is an essential protein for insect internal circulation because of its role in lipid transportation. In this study, we cloned and characterized ApoD and analyzed its expression characteristics in Bombyx mori (L.) (Lepidoptera: Bombycidae). The ApoD cDNA fragment, which has an open reading frame of 522 bp with a lipocalin domain (PF08212), encodes 174 amino acids. The deduced ApoD protein sequence has 28.2% identity to its ortholog in Mus musculus and 27.8% identity to its ortholog GLaz in Drosophila melanogaster. Several putative cis-acting elements related to development were also examined. Using quantitative real time polymerase chain reaction (RT-PCR), the ApoD gene expression was detected in the head and silk glands. Western blotting showed that ApoD protein was present in the head, hemolymph, fat body, gonad, middle silk gland, and posterior silk gland. ApoD expression was relatively higher in the hemolymph during days 1–3 of fifth-instar larvae and days 6–8 of the pupal stage. The highest mRNA levels were detected in the pupae on day 9 by using quantitative RT-PCR. After an oxidative stress treatment and a bacterial challenge, expression levels of the ApoD gene from B. mori were up-regulated at different time points compared with the control group. Simultaneously, immunohistochemical localization showed that ApoD is widely detected in the brain, eye, ovary, and testis on day 3 of the fifth-instar larval stage. The results indicated that ApoD is involved in insect development and reproduction, and plays an important role in the response to bacterial challenge and other stress signals.