Recent research has shown that calcium isotopes are fractionated by metabolic processes, leading to a decrease in 44Ca/40Ca ratio with increasing trophic level. If so, calcium isotopes could provide information on trophic relationships within foodwebs millions of years older than what we have been able to study thus far with alternative methods (i.e., nitrogen isotopes (δ15N), Sr/Ca). To explore whether δ44Ca values provided marine trophic level information, we measured the δ44Ca composition of tooth enamel and bone from modern marine mammals representing a 2.5 order range in trophic level. Marine mammal enamel δ44Ca values clustered into two groups—mammals foraging on vegetation or invertebrates exhibited higher δ44Ca values than those foraging on fish or other marine mammals. We next examined whether this correlation was preserved in the fossil record by examining a 15 Ma marine fauna from southern California and observed that the relationship between δ44Ca values of specimens followed the same pattern as observed in modern faunas, but the mean δ44Ca values were significantly different from modern δ44Ca values for mammals of similar trophic level. We conclude that the relative spacing of δ44Ca values amongst fossil taxa can serve as a valuable tool for defining trophic level of extinct organisms and can provide critical information on relationships within ancient foodwebs. Copyright © 2003 John Wiley & Sons, Ltd.