Cells of the achlorophyllous micro-alga Prototheca;zopfii were immobilized in 8-mm-cube polyurethane foam pieces. A 2-fold increase in the volumetric biodegradation rate of the immobilized cells for n-alkanes (mixture of C-14, C-15 and C-16) was observed compared with that of the immobilized system using calcium alginate gel in batch experiments using flasks agitated on a reciprocal shaker at 25 degrees C. The apparent biodegradation rates were influenced significantly by the affinities between algal cells and matrix and/or between hydrocarbons to be degraded and matrix. Such affinities resulted in the improvement of the interaction between the substrates and algal cells. The stability of the immobilized cells was examined in repeated-batch culture and activity was stable over three successive cycles of cultivation. P. zopfii immobilized in polyurethane foam was incorporated into a bubble-column type bioreactor for degrading hydrocarbons and the potential effectiveness of the immobilized cell system was confirmed.