Cry1Ac is one of the most-studied Bacillus thuringiensisdelta-endotoxins. Structurally, it is divided into two domains: the N-terminal half corresponding to the toxic component and the C-terminal half corresponding to the region responsible for the crystal formation. We engineered Cry1Ac delta-endotoxins modified in their N-terminal part and studied the effect of such modifications on crystallization and delta-endotoxin production. When expressed in an acrystalliferous B. thuringiensis strain, Cry1Ac(*) and Cry1AcDelta, variants with four point mutations and a deletion, respectively, could not form crystals. However, when expressed in a crystalliferous strain, these altered proteins were shown to interact with the endogenous delta-endotoxins and cocrystallize with them, forming atypical crystals observed by electron microscopy. This cocrystallization of the altered delta-endotoxins with the endogenous ones led to a decrease in delta-endotoxin production (27%) by the corresponding recombinant B. thuringiensis strains. This ability of altered delta-endotoxins containing an intact C-terminal part to cocrystallize with native ones could be exploited to promote the crystallization of foreign proteins by fusing them with the C-terminal part of Cry1A delta-endotoxins.