Ethylparaben was transesterified to methylparaben in the presence of methanol by Pseudomonas cepacia. When the bacterium was incubated in water-methanol solutions of ethylparaben in the range of 0.01 to 2% methanol, the higher the content of methanol, the higher the conversion ratios of ethylparaben. If over 5% methanol was used, the bacterium could not survive and the reaction did not proceed. Transesterifications also proceeded with 1-propanol or benzyl alcohol instead of methanol, though the conversion ratios were low. Conversion ratios were found to be higher throughout the conversion period by entrapping the cells to Ca alginate or polyurethanes than for free cells. The results are presumably due to the higher stability of cells in 10% alcohol in which free cells could not survive. PU-3 and PU-6 are urethane prepolymers consisting of polyetherdiol and phenylisocyanate segments. PU-3 has a high polypropyleneglycol content, while PU-6 is high in polyethyleneglycol (1).