The production and characterisation of transgenic tobacco plants expressing a precursor of a regulatory peptide from Aedes aegypti (Trypsin Modulating and Oostatic Factor, Aea-TMOF) which interferes with the development of tobacco budworm larvae is described. Tobacco plants were transformed with a synthetic gene containing 6 TMOF units spaced by dibasic residues, Arg-Arg, as potential post-translational cleavage sites. Peptide extracts from transgenic plants had TMOF activity and inhibited in vitro the biosynthesis of serine proteases. This activity was consistently present in T1 plants and absent in control plants. Tobacco budworm larvae, fed with transgenic leaves showed a reduced growth rate compared to those fed with control plants. The low rather than acute toxicity of this low impact gene is discussed in the context of alternative integrated pest management strategies.