Earlier investigations of the proteases that digest the blood meal in mosquitoes identified the midgut epithelial cells as the source of the main blood digesting protease, trypsin. Surgical manipulations and cytoimmunochemical studies indicated that trypsin in the mosquito is not stored in the epithelial cells as an inactive enzyme, but is synthesized, activated and released into the gut lumen after the blood meal. Using molecular biology techniques the major trypsin genes have been sequenced and some information on the transcription factors that regulate the induction of the genes have been reported. Although all the hormone(s) and the factors that are responsible for the induction of trypsin biosynthesis are not known, the termination of trypsin biosynthesis is controlled by a decapeptide hormone; trypsin modulating oostatic factor (TMOF) that is secreted by the mosquito ovary. Genetic engineering and expression of TMOF in bacteria, yeast and algae is emerging as a potential future larval control of mosquitoes in the field.