The hypothesis according to which multiple and different 5‐endotoxin genes could determine the host‐range specificity of the lepidopteran strains of Bacillus thuringiensis is being checked in the case of strains aizawai 7.29 (serotype 7) and entomocidus 601 (serotype 6). From these strains, several crystal protein genes, belonging to different structural types, have been isolated. One of the cloned genes that is not present in strain entomocidus 601 is duplicated in strain aizawai 7.29. This gene belongs to a previously characterized type of crystal protein gene and encodes a protein preferentially active against Pieris brassicae. Two other genes, of presumed chromosomal location, are present In both strains and each displays a unique physical map. In both strains the two genes are in close proximity and in the same orientation. The first, which belongs to a new type of crystal protein gene, encodes a 130–140 kD protein that is not significantly active against the two insect species tested. The other new type of crystal protein gene directs the synthesis of a polypeptide preferentially active against Spodoptera littoralis.