The previously defined anterior area (A) of guinea pig auditory cortex has been divided into a large dorsal portion identified as the primary area (AI) and a smaller ventrorostral belt (VRB). This division is based on: (1) the much longer response latency of units in VRB (21.7 ms) than AI (14.1 ms); (2) the absence of pure onset units in VRB (i.e. units that lacked a sustained response), which are common in AI; (3) the weakness of noise-induced evoked potentials in VRB compared to AI; (4) units in VRB had lower thresholds and stronger phase locking to amplitude modulated stimuli than in AI.