Abstract: Two genes encoding the predominant polypeptides of Bacillus thuringiensis subsp. thompsoni cuboidal crystals were cloned in Escherichia coli and sequenced. The new class of crystal proteins have electrophoretic mobilities of 40 and 34 kilodaltons (kDa) with the deduced amino-acid sequences predicting molecular masses of 35,384 and 37,505 daltons, respectively. No statistically significant similarities were detected for the 40 kDa and 34 kDa crystal proteins to any other characterized Bacillus thuringiensis crystal protein or to each other. A 100 MDa plasmid encodes both crystal protein genes, which appear to be part of an operon with the 40 kDa gene 64 nucleotides upstream of the 34 kDa gene. Both crystal proteins are synthesized in approximately the same amounts. Even though small, compared to other crystal proteins, the 34 kDa crystal protein has insecticidal activity against lepidopteran larvae (Manduca sexta).