Gibberella fujikuroi cells were immobilized on the surface of fibrous carriers with copolymers obtained by a radiation polymerization technique. The gibberellic acid (GA) production in the immobilized cells was investigated by varying the immobilization conditions and using various monomers. GA productivity in cells immobilized with the carrier-covered copolymer of hydrophylic hydroxyethyl acrylate and hydrophobic trimethylpropane triacrylate was higher than that in the free cells. The stability of the immobilized cells was evaluated by repetition of the batch fermentation. The immobilized cells produced GA with high stability. GA productivity by immobilized cells varied with the chemical nature of the carriers covered by the polymers.