Radiographic observation of four immature Nautilus pompilius shows that new chamber formation commences when approximately half of the cameral liquid of the last formed chamber has been osmotically removed. At this time, the septal mantle is rapidly moved forward in the body chamber, where it reattaches to the internal shell wall and begins calcification of a new septum. Throughout this time, apertural shell growth rates remain constant. The time between successive chamber initiations increased during ontogeny; in the four specimens monitored here, time between two successive chamber formation events ranged between 85 and 132 days and showed no apparent lunar correlation.