The objective of this study is to investigate the feasibility of developing cellulose acetate (CA) membranes to partially immobilize Pseudomonas putida (ATCC 49451) and to evaluate the inhibitory effect of phenol on the immobilized bacteria by monitoring their growth in partially immobilized and free-suspension systems. The cellulose acetate membranes used in this study were wet spun from 20 wt % of CA in 1-methyl-2-pyrrolidone (NMP)/acetone (30 : 70) solvent using water as the bore fluid as well as the external coagulant. Scanning-electron microscopy (SEM) characterization of the newly developed CA hollow fibers suggests that the fiber cross section consists of multilayer microporous structures useful for cell immobilization. Experiments were conducted using the bacteria to degrade phenol at initial phenol concentrations of 300 mg/L and 1000 mg/L. In a free suspension (no membrane) system, it was observed that the bacteria were able to grow optimally at 300 mg/L of phenol, and degraded phenol almost completely in about 26 h. However, neither cell growth nor phenol degradation occurred when initial concentration of phenol was increased to 1000 mg/L. In a cell-immobilized membrane system, the cell growth and phenol concentration profiles in the medium were very similar to those obtained in a free suspension culture if phenol concentration was 300 mg/L. However, when the initial phenol concentration was increased to 1000 mg/L, data obtained in a cell-immobilized membrane system were discernibly different from that obtained in the suspension culture. In the former case, phenol concentration decreased in the beginning of test, indicating that the carbon source has been consumed and immobilized cells within the membrane had begun to multiply. As soon as the phenol concentration decreased to about 700 mg/L (at which concentration, substrate inhibition was not as severe as 1000 mg/L), partial immobilization occurred when some cells diffused out of the membrane into the medium and optical density became measurable in the medium. It was found that cell growth continued for the next 32 h, reaching an optical density in the medium of 0.42 absorbance units and a significant amount of phenol was degraded.