Marine diatoms are an important food resource for bivalves, but few experimental studies have evaluated diatom assimilation by bivalves. We conducted a laboratory experiment to investigate the ability of the suspension-feeding bivalve Nuttallia olivacea to utilize three common diatom species (planktonic diatoms Thalassiosira pseudonana and Skeletonema dohrnii and the benthic diatom Entomoneis paludosa) as food labeled with heavy nitrogen stable isotope (¹⁵N) by incubation in medium containing Na¹⁵NO₃. The percentage of food-derived nitrogen in the organs of the bivalves increased over time, confirming that the bivalves were taking up dietary nitrogen from diatoms. The proportion of food-derived nitrogen from diatoms to bivalves appeared to be higher in planktonic species than in benthic species. However, it is possible that the benthic diatom intake by the bivalves in this study was underestimated because the substrate was not disturbed as would occur under field conditions. The percentage of food-derived nitrogen in bivalve organs tended to be highest in the digestive diverticula, followed by the foot, mantle, and siphon, regardless of diatom type. These findings suggest that N. olivacea may preferentially distribute nitrogen to organs other than the siphon, which is prone to continuous loss by fish predation.