The yeast Saccharomyces cerevisiae was entrapped within polyacrylamide gel beads by employing a procedure that uses sodium dodecylsulfate as a detergent to improve the spherical configuration of the beads. The resulting preparation showed a rate of fumarate bioconversion to L-malic acid about 60 times higher than that found for the free cells. Almost all fumarate was converted in 30 min of incubation. The thermal stability of the immobilized cells did not significantly differ from the free cells. An optimal pH of 5.7 was found for the immobilized preparation and no succinic acid was detected as a byproduct in the incubation mixture.