We have isolated and characterized a nuclear gene, MSS18, which is implicated in the splicing of intron aI5 beta of the mitochondrial cox1 transcript (subunit 1 of the cytochrome c oxidase). Northern blotting and S1 nuclease protection experiments as well as the analysis of mitochondrial point revertants suggest that mss18 mutations block (perhaps indirectly) the cleavage of the 5' exon-intron junction of aI5 beta. Mitochondrial point revertants also indicate that up to 13 bases of the aI5 beta exon sequence, upstream of the 5' splice site of aI5 beta, are involved in vivo in the splicing of this intron. The implications of this result on the splicing of group I introns are discussed.