The pennate Diatom Haslea ostrearia was successfully immobilized in agar-gel disks and tested for marennine production in batch experiments. The concentration of pigment recovered varied between 34 and 92 mg l(-1) liquid medium, according to the initial cell loading of the gel structures, which corresponded to volumetric production rates between 15 and 50 mg day(-1) l(-1) gel. Average specific productivities of immobilized cells were higher than those of free cells. Immobilized-cell preparations could be used three times (working time > 1000 h) without significant loss in production efficiency.