Differences of u p to 5 % in bI3C and b15N were consistently observed across a size spectrum (< 3 to > 250 pm) of suspended particulate organic material (SPOM) sampled from a coastal Mediterranian site over a 10 mo period. On each of 4 sampling dates, lowest values of b13C and 615N (to -25.3 and -0.5 %o, respectively) were always found in size fractions < 8 pm, with higher values in larger particles Relationships among SPOM 6I3c, h'", and C/N suggest that isotope fractionation associated with plankton C and N cycling is responsible for the isotop~c var~ations observed. The relative I3C and 15N depletions present in the sn~allest but most abundant SPOM size fractions are consistent with the view that nano-and picoplankton constitute a sizable, low-trophic-level component of marine SPOM.