Using ⁶⁸Ge:¹⁴C dual-labeling, we investigated the absorption efficiency of diatom carbon for the cold water appendicularian Oikopleura vanhoeffeni. The absorption efficiency of bulk carbon (mean = 67%) was not influenced by body size or ingestion rate. For the first time for a pelagic tunicate, food and feces were fractionated into their major biochemical constituents (i.e., low-molecular-weight compounds, lipid, protein, and polysaccharide), allowing calculation of absorption efficiencies for each fraction. Low-molecular-weight compounds and proteins were pref-erentially absorbed over lipids and polysaccharides. However, predicted C: N ratios of the fecal pellets of O. vanhoeffeni were in the lower range of C: N ratios reported for zooplankton feces. The results are relevant for modeling biogeochemical cycles because pelagic tunicates contribute greatly to vertical particulate organic carbon flux.