Nautilusr pompilius and N. macromphalus shells show, under cathodoluminescence microscopy. a zoned luminescence of yellow to green for the former and blue to blue-green for the latter. Cathodoluminescence results from fluctuations in the amount of manganese present in the aragonite. There is a relationship between this phenomenon and growth lines. Luminescence intensity increases with ontogeny. Variations in the metabolic activity appear to be linked with the manganese content of the shell. Environmental factors may have an effect by way of their repercussion on the metabolic activity of Nautilus. The Mn2+ amount may be influenced by the physico-chemical pattern of the surrounding water but it appears also to be controlled at the species level.