Carbon and oxygen isotope ratios have been determined for 191 fossil echinoid specimens ranging in age from Devonian to Pleistocene. The data for these specimens, representing 45 families and 16 orders, are compared with published C-13/C-12 and O-18/O-16 measurements of skeletal calcites from 256 Recent echinoids representing 28 families. Both diagenetic alteration and genetic control of isotopic composition are evident in the data for fossil species, but the latter predominates. Variability in isotope ratios increases from Devonian to Recent and parallels the increase in evolutionary diversity. Although the number of analyses is too small to assess evolutionary changes in detail, results indicate that Paleozoic echinoids had higher C-13/C-12 and lower O-18/O-16 than did post-Paleozoic echinoids.