Although several cortical areas other than the primary area (A1) have been delineated in the auditory cortex in a number of mammalian species, the organization of auditory cortices remains to be fully revealed.Using a real-time imaging technique, we investigated the organization of auditory cortices in mice, and identified a novel field in the insular cortex (IAF).IAF responded to pure tone stimulation earlier than AI and showed preference to tones of lower frequencies.Prompted by the early response of IAF, we examined its thalamic input and found that IAF received lemniscal input parallel to the input to A1.Our results thus reveal a lemniscal auditory pathway to the insular cortex that preferentially processes sounds of lower frequencies.