A new selectable marker for transformation of Dictyostelium discoideum cells was constructed by using the bsr gene from Bacillus cereus, which confers resistance to Blasticidin S. The bsr gene was driven by Dictyostelium actin 15 promoter and Dictyostelium actin 8 terminator for expression in Dictyostelium cells. To demonstrate the feasibility of using the bsr marker, we constructed an extrachromosomal replication vector by replacing the Neor gene of pnDeI (B. Leiting and A. Noegel (1988) Plasmid 20, 241-248) with the bsr gene cassette. A mutant Dictyostelium actin 15 gene was constructed and inserted into the vector. Dictyostelium cells were transformed with the resulting vector and then transformants were selected with Blasticidin S. The selected cells showed high level expression of the mutant actin, indicating an efficient selection of transformed cells with the bsr marker.