When the active form of the Bacillus thuringiensis delta-endotoxin Cry1Ca was expressed in E. coli severe growth retardation was observed. The absence of a short peptide from the N-terminus of the protoxin was responsible for this effect. The introduction of a mutation at an amino acid previously reported as being involved in the initial stages of pore formation within the natural insect target partially abolished the growth retardation effect. We suggest that removal of the N-terminal peptide is a necessary step in toxin activation, the presence of this peptide preventing proper interaction of the toxin with the target membrane. Expression of the truncated toxin in Bacillus thuringiensis also prevented the formation of Cry1Ca crystals.