Aphids are sap-sucking insects (order: Hemiptera) that cause extensive damage to a wide range of agricultural crops. Our goal is to produce a "designer" toxin based on naturally occurring insecticidal crystalline (Cry) toxins produced by the soil-dwelling bacterium Bacillus thuringiensis for use against the pea aphid, Acyrthosiphon pisum. On the basis that activation of the Cry4Aa protoxin could be a rate-limiting factor contributing to the relatively low aphicidal activity of this toxin, we introduced cathepsin L and cathepsin B cleavage sites into Cry4Aa for rapid activation and generation of the active toxin in the aphid gut environment. Incubation of modified Cry4Aa and aphid proteases in vitro demonstrated enhanced processing of protoxin into the active form relative to non-modified protoxin. Aphids fed artificial diet with protoxin at a final concentration of 125 µg/ml showed enhanced toxicity after two days for some of the modified constructs. This is the first study that has facilitated Cry protoxin activation to achieve toxicity against pea aphids. This work highlights the potential for the use of designer toxins for managing aphid populations via transgenic plant resistance.