Light scattering properties of biogenic CaCO3 particles were determined on cultured calcifying algae and field-derived CaCO3 particles. The particles were separated from particulate organic carbon (POC) with a flow cytometer, volume-scattering functions were measured with a laser light-scattering photometer, and particle composition was measured using atomic absorption spectrometry. Small calcite coccoliths were best sorted by gating on the ratio of horizontally polarized forward light scattering and vertically polarized forward light scattering; plated coccolithophores could be sorted by gating on side scattering and forward angle light scattering. Normalized volume-scattering functions for the culture-derived calcite particles varied by a factor of 2 for the different species. Backscattering cross sections (m2 particle-1) for calcite particles varied by ~35 times and were generally a function of size. Backscattering efficiencies were ~2--4 times higher for cells with CaCO3 than without it. CaCO3-specific backscattering showed much less variability across various species; the calcite-specific backscattering coefficient varied by only ~38% for both cultured coccolithophores and field-derived CaCO3 particles. Organic carbon-specific backscattering of naked coccolithophores was highly consistent within all coccolithophores used in our experiments, as well as with values in the literature. Our results suggest that both POC and PIC can be optically estimated, the former by measuring backscattering of decalcified phytoplankton as well as their size distribution, and the latter is proportional to acid-labile backscattering. These results show the feasibility of a rapid optical technique for measuring two biogeochemically important carbon fractions in the sea. ¿ 1999 American Geophysical Union