Cetaceans diverged from terrestrial mammals approximately 53 mya and have evolved independently since then. During this time period, they have developed a complex nervous system with many adaptations to the marine environment. This study used stereological methods to estimate the total number and diameter of the myelinated fibers in the corpus callosum of the common minke whale (Balaenoptera acutorostrata) (n= 4). The total number of callosal fibers was estimated to 55.3 × 106 (range: 49.0 × 106–59.1 × 106). Despite large variations of the callosal area (350–950 mm2), there was little variation in total fiber number. The fibers with diameters ranging from 0.822 to 1.14 μm were the most frequent, which is similar to results obtained in the human brain using the same method. There was no systematic distribution of large-, middle-, or small-sized fibers along the rostrocaudal axis of the corpus callosum. This study indicated that the corpus callosum of the common minke whale is small and has few fibers compared to terrestrial mammals.