Negative bias in the perception of facial emotion is one of the remarkable symptoms of depression. However, little is known about the neural mechanisms underlying the bias. The anterior cingulate cortex (ACC) has been implicated in depression. Structural and functional imaging studies have found a decrease in volume and activity of the perigenual ACC. To date, however, whether or not neurons in the ACC are involved in the perception of facial emotion is unknown. In the present study, we recorded the activity of single neurons from the anterior part of the ACC of two rhesus monkeys and examined the responsiveness of the ACC neurons to various visual stimuli including monkey faces, foods, artificial objects, and snakes. About 20% of recorded neurons showed a significant increase in activity in response either to monkey faces or to other categories. Among the responsive neurons, the proportion of the neurons responsive to monkey faces was significantly higher. Some neurons showed differential responses to different facial emotions. Most of the responsive neurons were located just above or anterior to the genu of the corpus callosum, that is, the perigenual portion of the ACC. The ACC has a strong and mutual connection with the amygdala. These results suggest that the neural network, including the ACC and amygdala, is involved in the processing of facial emotion, and that the negative bias in the perception of facial emotion might be induced by dysfunction of this network.