The cyanobacterium, Aphanocapsa halophytia MN-11, was immobilized in calcium alginate gel and coated on light-diffusing optical fibers (LDOF) for sulfated extracellular polysaccharide production. Results indicated that sulfated extracellular polysaccharide production depends on the number of immobilized cells and the light intensity. In addition, the production rate reached 116.0 mg (mg dry cells)(-1) day(-1) when the cells that were immobilized on LDOF were incubated under a light intensity of 1380 ed sr m(-2) at a cell concentration of 1.0 x 10(8) cells/cm(3) gel. Cells immobilized on LDOF produced about ten times more sulfated extracellular polysaccharide than those immobilized in calcium alginate beads only (11.7 mg(mg dry cells)(-1) day(-1)).