A petroleum-degrading achlorophyllous micro-alga, Prototheca zopfii, was immobilized in calcium-alginate beads and the rate of degradation for n-alkanes (C-14, C-15 and C-16) measured. The rate of biodegradation for the hydrocarbon mixture could be described by a second-order rate expression which is a function of both population of algal cells and substrate concentration. The volumetric biodegradation rates observed in free cells were comparable to those reported in other systems of marine petroleum-degrading microorganisms, such as Bacillus sp. and Pseudomonas sp. A modified bacterial adherence to hydrocarbon assay revealed the strong hydrophobicity of the outermost surface of the algal cells. Repeated cycles of subculture were carried out and the stability of the immobilized algal cells for biodegradation confirmed.