The MRS3 gene cloned in the multicopy plasmid YEp13 suppresses the mitochondrial splice defect exerted by mutation M1301 in the group II intron bI1. In this article we report on the behavior of the MRS3 gene cloned in the integration vector pEMBLYi27 and in the CEN4-ARS vector YCp50. Transformation of mutant M1301 cells with these recombinant vectors produced transformants, the majority of which showed the original splice defect and contained the recombinant vectors in single or low copy; a minority, however, was splicing competent and showed exceptionally high copy numbers of the MRS3 gene. These latter transformants had either the pEMBLYi27/MRS3 sequence repeated at least 20 times in tandem at the chromosomal site of the MRS3 gene or they had the YCp50/MRS3 sequence established as a multicopy plasmid lacking the copy number control usually exerted by the CEN4 sequence in this plasmid.