Lead-210 activities in carefully cleaned size groups of pteropod and heteropod shells from the eastern equatorial Pacific were typically 0.3 dpm/g shell, similar to those reported in other biogenic calcium carbonates. However, 210Po activities were in excess of levels expected from the decay of shell-incorporated 210Pb, with (210Po/210Pb) activity ratios ranging from 20 to 28 in size-fractionated samples. Sample treatment procedures were examined using pteropod shells collected from diverse locations of the North Pacific. Possible sources of 210Po in excess of 210Pb in shells include inclusion of 210Po in the organic matrix during shell formation or adsorption of 210Po from the ambient water or nuclide-rich animal tissues. We predict that other CaCO3-precipitating organisms, including foraminifera, that have high surface/volume ratios and tissues in contact with carbonate surfaces, may also contain 210Po in excess of that supported by the decay of 210Pb. Variation in 210Pb activities measured in pteropod and heteropod shells suggests differences in the depth distributions for the species analyzed. Moreover, these results suggest ontogenetic migration in the pteropod Cavolinia longirostris. Pteropods and heteropods from regions in the equatorial Pacific were enriched 8–15 times with 210Pb relative to calcium, based on reported 210Pb activities in surface water and seawater calcium concentrations. Similarly, enrichment factors calculated for pteropod shells from the North Pacific transition zone and Subarctic Current were 13–20 times 210Pb levels in surface waters. Skeletal enrichment of 210Pb is higher in pteropods and heteropods than in corals by a factor of 3–10.