The invention relates to a mutant of Bacillus thuringiensis which produces a larger amount of crystal delta-endotoxin with a greater pesticidal activity as compared to the corresponding parental strain. The mutant may also have a larger crystal size as compared to the corresponding parental strain. The crystal delta-endotoxin produced by the mutant Bacillus thuringiensis will have an activity directed towards the same pest(s) as its parental Bacillus thuringiensis crystal delta-endotoxin. The invention further relates to a method for producing such a mutant, compositions comprising such a mutant as well as methods for controlling a pest(s) using these compositions.