The sdhB gene encoding an iron-sulfur (Ip) subunit of succinate dehydrogenase (SDH, EC 1.3.99.1) complex was cloned from Mortierella alpina 1S-4. The deduced amino acid sequence of SdhB from M. alpina 1S-4 showed high similarity to those of SdhB from other organisms. The mutated sdhB (CBXB) gene encodes a modified SdhB with an amino-acid substitution (a highly conserved histidine residue within the third cysteine-rich cluster of SdhB replaced by a leucine residue) and is known to confer carboxin resistance. We succeeded in transforming M. alpina 1S-4 by using the CBXB gene as a selectable marker gene and expressing the heterologous uidA gene encoding beta-glucuronidase of Escherichia coli. Moreover, transformation efficiency was up to 40-50 transformants per 4.0 x 10(8) spores. This carboxin-transformation system, characterized by marginal background growth and mitotic stability in M. alpina 1S-4, is considered to be widely useful for the wild strain, M. alpina 1S-4, and various derivative mutants without laborious preparation of auxotrophic mutants as a host strain.