In the romboencephalon of a developing viviparous Teleost (Gambusia affinis), the mitotic activity slowly decreases and falls at hatching, but rises in young fish; therefore mitoses are present till the end of somatic growth. The mitotic activity is higher in the alar plate than in the basal one: this is due to the late differentiation of the sensitive region. During the growth of the romboencephalon, mitoses rarefy rostrally and, in the oldest embryos they are present only near the calamus owing to a number of undifferentiated elements in this area. In the developing romboencephalon of Gambusia not many extraventricular mitoses are present, because of a wide ventricular surface allowing ventricular migration. This confirms that the high number of extraventricular mitoses found in the developing spinal cord of the same Teleost is a typical phenomenon due to the quick reduction of the central canal. Therefore ventricular migration may be hampered by physiological conditions during normal development.