We measured the respiratory isotope effect &egr;resp for seven representative unicellular marine organisms. The bacterium Pseudomonas halodurans, the diatom Phaeodactylum tricornutum, the phytoflagellates Cryptomonas baltica and Dunaliella tertiolecta, the heterotrophic flagellates Paraphysomonas imperforata and Bodo sp., and the ciliate Uronema sp. exhibit &egr;resp values in the range 14--260/00. We also measured &egr;resp for three metazoans. The &egr;resp for the copepod Acartia tonsa ranged from 17 to 250/00, while two larger organisms, the mollusk Mercenaria mercenaria and the salmon Salmo salmar, respire with a smaller &egr;resp of 5--100/00. The average respiratory isotope effect of the dominant marine respirers (the bacteria, microalgae, and zooplankton) is about 20¿30/00. An &egr;resp of this magnitude supports the hypothesis that the photosynthesis-respiration cycle is responsible for the 23.50/00 enrichment in the &dgr;18O ratio of atmospheric O2 relative to seawater (the Dole effect). The large value and high variability in the average &egr;resp limits the usefulness of a proposed method using the &dgr;18O of naturally fractionated dissolved O2 in seawater as a tracer of primary production in the oligotrophic ocean. ¿ American Geophysical Union 1993