This study shows that the output mass concentration of respirable cotton dust produced by the Pitt-3 acoustical particle generator falls off rapidly with time when the generator is driven at the recommended voltage and frequency of 6 V RMS and 60 Hz. The object of this investigation was to determine if the rate at which the output mass concentration decreased could be reduced by operating the generator at its resonant frequency. It was shown experimentally that resonance of the generator system occurred at approximately 30 Hz and when the generator was operated at that frequency the output mass concentration decreased less in a 6 hr period than it had previously decreased in 40 min at 60 Hz. These results suggest, therefore, that in cases where a stable concentration of cotton dust is needed over an extended time period for instance, during animal exposure studies, it would be much better to operate the Pitt-3 generator near its resonant (natural) frequency.