Frequency-Tuning in 46 high-firing single units from primary auditory cortex of ketamine-anaesthetized cats was studied separately for isolated-spike firing and burst spike firing. The tuning curve for burst firing was significantly narrower than for the overall firing rate condition and, consequently, much narrower than for the isolated spikes. The burst firing was found in excess of that predicted from a modulated Poisson process, so the results cannot be explained on basis of a change in firing rate criterion. We propose that burst firing in auditory cortex may selectively enhance frequency tuning dynamically controlled by the behavioural state of the animal as well as its stimulus history. Thus burst firing may have functional implications for cortical processing.