Working memory is thought to depend on sustained activity in prefrontal cortex (PFC) neurons. Here, spiking and local field potentials were recorded on a trial-by-trial basis from PFC neurons in monkeys trained in a working memory task. During the delay between stimulus and response there were bursts of spiking in individual neurons, increased gamma oscillations and suppressed beta oscillations, suggesting that working memory is encoded by bursts of activity rather than by sustained activity.