To discern controls on particulate organic carbon (POC) : natural radionuclide (RN) ratio variability in order to enhance the accuracy of water column radionuclide-based carbon flux estimates, 234 Th, 210 Po, and POC were analyzed in seven size classes of mixed micro-and mesoplankton (1-1,500-mm size range), in larger zooplankton from different taxa (salps, euphausiids, copepods, pteropods), and in freshly produced feces from zooplankton collected during spring in the NW Mediterranean. POC : RN ratios in zooplankton ranged between 120 and 11,600 and between 89 and 9,200 mmol dpm 21 for 234 Th and 210 Po, respectively. In fecal pellets, POC : RN ratios were one to three orders of magnitude lower for 234 Th and 3-fold to 30-fold lower for 210 Po; the only exception was euthecosome pteropods, which had a higher POC : 210 Po ratio in their pellets than in their whole bodies. Significant increases in POC : RN ratios with organism size were best described by a power relationship for POC : 234 Th (p , 0.0006) and a saturation exponential equation for POC : 210 Po (with a constant POC : 210 Po ratio above 70 mm; p , 0.004), suggesting that the observed trend most likely results from surface adsorption processes for 234 Th and food chain bioaccumulation for 210 Po. This inference is further supported by the observation that, for the .33-mm size classes, 210 Po specific activity correlates negatively with the surface : volume ratio, while 234 Th correlates positively with it (p , 0.004 and p , 0.001, respectively). POC : RN ratios vary greatly among species and to a lesser extent among fecal pellet types, most probably due to differences in zooplankton feeding strategies. Partial removal of most zooplankton ''swimmers'' from trap samples would not likely confound assessment of 234 Th flux; however, it could considerably bias similar measurements of 210 Po flux as well as those of POC : RN ratios.Given the interest in climate change and the potential role of the oceans in sequestering carbon, 234 Th and to a minor extent 210 Po have been increasingly used to assess particle fluxes, especially particulate organic carbon (POC) out of surface waters (e.g., Kim and Church 2001;Friedrich and Rutgers van der Loeff 2002;Murray et al. 2005). When using natural radiotracers such as 234 Th and 210 Po to study carbon export and cycling in the oceans, it is necessary to determine both the activity concentration of the radionuclides in the water column and their ratio to carbon in the particles responsible for carbon sequestration and export (Moran et al. 2003).Although information on POC : 234 Th ratios in a variety of particles from different size classes and regions is available (for review, see Buesseler et al. 2006), similar data on POC : 234 Th and POC : 210 Po in zooplankton and fecal pellet material are extremely sparse (Fowler and Fisher 2004). This is somewhat surprising, considering that (1) many studies show that POC : 234 Th variability is much greater in surface waters than at depths below the euphotic zone (Bu...