Inoceramid bivalves have been interpreted as both benthic and pseudoplanktonic. The comparison of 18O/16O and 13C/12C signatures of inoceramid shells with surface-dwelling and bottom-dwelling organisms should provide a simple means of resolving the controversy; however, we have found that the stable- isotope pattern is ambiguous. The bivalves have oxygen values similar to their contemporary benthic foraminifera but have carbon values similar to their contemporary planktonic foraminifera. We attempt to resolve this paradox by interpreting those inoceramids analyzed as benthic organisms that harbored chemosynthetic symbionts. A similar pattern of heavy oxygen and heavy carbon values is found in shell carbonate of some modern bivalves with chemosynthetic symbionts living around cold seeps off the coast of Oregon.