Translational lacZ fusions to the promoters of the parasporal, crystal protein (binary toxin) and 100-kDa ADP-ribosylating mosquitocidal toxin genes of Bacillus sphaericus were prepared and expression of the toxin genes monitored as ß-galactosidase activity. Transcription of the crystal protein gene fusion began immediately before the end of exponential growth and continued into stationary phase in both B. sphaericus and Bacillus subtilis but accompanied exponential-phase growth in Escherichia coli. Expression of this fusion was severely delayed in a B. subtilis spo0A mutant and decreased relative to the wild type in a B. subtilis spoIIAC background. ß- Galactosidase activity from the 100-kDa toxin gene fusion was restricted to early exponential phase in B. sphaericus, but in B. subtilis it continued into late exponential phase. Expression was about eightfold lower in B. sphaericus than B. subtilis suggesting an element of negative control in the native host.