Nacella concinna (Strebel, 1908) held in seawater enriched with Sr, partially replaced Ca in their shells with Sr The bands laid down were clearly visible In back-scattered, scanning electron microscope images, and such bands could be used to mark carbonate skeletons in field investigations of growth Sr directly replaced Ca in the carbonate shells, although not entirely, and newly secreted shell material always contained less Sr than would be expected from the levels used in treatments. Specimens held in water with atomic Ca:Sr ratios of 114:1 (control seawater, no added Sr), 4.4:1, 2.2:1 and 1.1.1 produced shell ratios of ca. 635:1, 11.0.1, 3.1:1, and 8.1:1 respectively These values were between 1.4 and 7 4 tunes higher than expected, indicating that, when shell was laid down Sr was discriminated against by a factor of around 4. Growth rates were similar before and after placing specimens in experimental treatments and feeding rates were not significantly different between treatments and controls, indicating that the enhanced Sr regimes caused little stress for the limpets. Limpets were exposed to the experimental treatments for 12 days. However, in specimens where growth bands were visible, it was only possible to distinguish between 5 and 8 micro-growth lines. In specimens removed after 3 days exposure, there was no detectable area of enhanced Sr in the shell. These two observations were interpreted as either meaning that it takes around 5 days for Sr to enter the extra-pallial fluids which are used for laying down shell, or that the limpets did not produce any shell growth during that period. Experiments placing Sr in food and not in the treatment water, and vice-versa, suggested that Sr taken directly from the water may be more important than uptake from food.