Cell surface and intracellular accumulation kinetics of phenanthrene were studied in two coastal marine diatoms. Cell surface uptake and depuration rate constants were two to three times greater in the smaller Thalassiosira pseudonana than in T. weissflogii, reflecting the 2.8-fold difference in surface-area-to-volume ratio (A/V) between the two species. However, cell surface accumulation was faster than most other environmental and biological process, supporting the assumption that cell surfaces are in equilibrium with the water. Intracellular depuration rate constants of phenanthrene were similar (0.56 d(-1), 0.62 d(-1)) for both diatoms and for other hydrophobic organic contaminants (ca 1 d(-1)) with a range of hydrophobicities and chemical structures in other microalgae. Biodilution could be a significant factor in the internal accumulation of phenanthrene, as phenanthrene loss rate constants were on the order of phytoplankton growth rates in the spring and summer (1-3 d(-1)). Organic carbon-normalized phenanthrene bioconcentration factors were not significantly different for the two diatom species. The fraction of phenanthrene in the cell surface compartment at steady-state (x1) was directly related to the A/V of the two diatoms and a Chlorophyte microalga, showing that intracellular partitioning, which may affect phenanthrene trophic transfer, depends on phytoplankton cell size.