Populations of calcium-alginate immobilized marine diatom Phaeodactylum tricornutum were exposed to two sediments containing different levels of surfactant (LAS). Toxic responses were compared for free and immobilized algae. Although there is a direct relation between LAS content in sediment and inhibition, immobilized algae suffered less inhibition than free cells, over all when fluorescence is chosen as a biomarker for biomass. When cells are counted from dissolved beads, inhibition of growth is closer to the values found for free cells. Immobilization can be useful for in situ experiments but protection of cells inside the alginate beads against toxic capacity of xenobiotics must be taken into account.