Hemispheric fine-grain maps of primary auditory cortex (AI) were derived from microelectrode penetrations in the temporal gyrus of the common marmoset (Callithrix jacchus) to 1) compare the functional organization of AI in the marmoset with other mammalian species and 2) compare the right and left AI maps in individual monkeys. Frequency receptive fields (FRFs) were recorded with pure tones. Five FRF parameters were analyzed: characteristic frequency, threshold, sharpness of tuning 10 dB and 40 dB above threshold, and minimum response latency. The present study confirms that the functional organization of AI is characterized by a robust tonotopic frequency gradient overlaid with spatially clustered distributions of other FRF parameters. Globally, this functional organization of AI in the common marmoset is similar to that in other mammalian species. With respect to within-subject hemispheric comparisons of the five FRF parameters, a coherent pattern of asymmetry is not evident in marmoset AI. The overall results indicate that the basic functional organization between hemispheres is similar but not identical. J. Comp. Neurol. 487:391–406, 2005. © 2005 Wiley-Liss, Inc.