Summary The nucleotide sequence of a 2711 bp DNA segment which contains the N ‐terminal coding sequence and the 5’flanking region of a crystal protein gene ( bta ) from Bacillus thuringiensis subsp. aizawai 7.29 has been determined. The coding region encodes an 824 amino‐acid polypeptide corresponding to a carboxy‐terminally truncated δ‐endotoxin specifically active against the cotton leaf worm Spodoptera littoralis. Comparison of the deduced amino acid sequence of the bta gene with that of the 4.5, 5.3 and 6.6 kb classes of lepidopteran‐active β‐endotoxins revealed that the Bta sequence contains a very high level of amino acid substitutions in the N ‐terminal part of the protoxin molecule. The substitutions are grouped in several highly variable segments separated by highly conserved regions. These conserved domains are also present in the dipteran‐ and coleopteran‐active δ‐endotoxins. The control region of the bta gene shows considerable DNA identity with the control regions of the other lepidopteran‐active genes. Deletions of the 3’region of the gene were carried out and the toxic fraction of the bta δ‐endotoxin was identified with the N ‐terminal half of the molecule.