Among insects, the melanisation immune response is an important part of their innate immunity. Current research has provided evidence that insects regulate their melanisation immune response via the activities of inhibitors, such as the serine protease inhibitors. However, as the key substrate of the melanisation process, it is plausible that dopamine may also play a role in regulating the strength of the melanisation immune response. To address this hypothesis, a novel transgenic RNAi-inhibited gene system was developed to suppress dopamine catabolism in the model lepidopteran insect, the silkworm Bombyx mori, and later challenged the resulting transgenic insects with the bacterium Escherichia coli. Furthermore, comparisons were made on the expression levels of dopamine related genes amongst the transgenic and wild type silkworms in two of the major insect immune tissues, fat body and haemolymph, using real-time PCR and Semi-quantitative PCR. Results from this study showed that when E. coli germsolution was injected into the silkworms, the concentration of dopamine was markedly increased in the transgenic compared with the wild type group. The expression of genes BmTH and BmDDC, which are involved in the anabolism of dopamine was suppressed while the expression of genes BmEbony and Bm-iAANATs, which are involved in dopamine catabolism was elevated in transgenic silkworms injected with E. coli. These findings suggest that dopamine may play an important role in regulating the strength of the melanisation immune response. Therefore this study contributes to the current knowledge of melanisation immune response in insects.