In vivo crystallization of the Bacillus thuringiensis insecticidal protein toxin Cry2Aa has been studied in the presence and absence of its associated crystallization factor Orf2, When expressed in the same cell, both proteins were localized to the cuboidal cytoplasmic crystal. When expressed in the absence of Cry2Aa, Orf2 was randomly distributed throughout the cytoplasm. Cry2Aa when expressed in the absence of Orf2 did not form visible crystals but could be recovered from the insoluble fraction of the cell lysate and solubilized at high pH. Purified Orf2 was found to be soluble over a wide range of pH although it could be co-precipitated in the presence of Cry2Aa, suggesting a direct interaction between the two.