The growth rates of ammonites and extinct nautiloids have been estimated in two ways: through analyses of shell growth lines and by analyzing the patterns of oxygen isotopic values from successive septa. In both types of studies, it has been assumed that the amount of time between successive chamber formation events is approximately constant. This assumption has never been tested with living cephalopods, however. To examine this, 10 immature Nautilus pompilius and two immature N. macromphalus were maintained in a surface aquarium for a period of 1 yr and periodically radiographed. The radiographs allowed direct observation of chamber formation events and apertural shell growth. During this observational period 61 separate chamber formation events were observed in the nautiluses. The time between separate chamber formation events increased in successively produced chambers, and varied from a minimum of 2–3 wk in a specimen of 45-mm shell diameter to a maximum of 13–15 wk in a specimen of 132-mm shell diameter. Unlike interval of chamber formation, which increased during ontogeny, rate of apertural shell growth showed no observable rate increase or decrease during ontogeny prior to maturity. With the onset of maturity, as marked by the shell characteristics defined by Collins et al. (1980), apertural shell growth rates dropped markedly, and ceased coincident with the removal of the last volumes of cameral liquid in the last formed, approximated chamber. Both rate of apertural shell growth and septal spacing were affected by degree of shell breakage.