To investigate the post-hatching development of the brain in a benthic octopod, Octopus ocellatus, we performed volumetric analyses of the brain. The brain consisting of the supra- and subesophageal masses was divided into 5 regions according to the functions suggested for the brain of another benthic octopod Octopus vulgaris, and the volume of each region was estimated at three post-hatching ages. We found that the inferior frontal lobe system and the brachial lobe increased in relative volume as the animals grew, while the basal lobe system decreased in relative volume. This result suggests that increasing demand for processing tactile information after hatching is reflected in the higher developmental rate in the centers devoted for tactile sense and related learning. We also found that the inner neuropile layer mainly consisting of dendrites, synapses and axons showed great increases in volume compared with the outer neural-cell-body layer. Although the increase in volume of the inner layer was marked during 1 month after hatching in all brain regions examined, the extent of the increase varied among brain regions. Developmental changes in cell densities in the outer layer also varied among the regions. The present results suggest that the post-hatching development of the brain in O. ocellatus is not homogeneous but varies among brain regions depending on different roles in controlling the behavior.