The effects of mutations have been studied within the apocytochrome b gene on the processing of transcripts from the gene for subunit 1 of cytochrome c oxidase (coxI). Most mutations which affect the expression of the reading frame encoded by the fourth intron of the apocytochrome b gene (bI4) result in a failure to remove intron aI4 from precursor transcripts of coxI. Mutations in other apocytochrome b introns result in additional and complex defects in the processing of subunit I transcripts. Mutants M1233 and M1282 are mutated within the second intron (bI2) of the apocytochrome b gene and have OXI3 transcripts of 4900, 6100, and 6500 nucleotides. These transcripts are absent from the wild-type strain and do not hybridize with all exon sequences of this gene. In mutant M1392 (mutated within the third intron of the apocytochrome b gene), two OXI3 transcripts of 2200 and 2800 nucleotides are present which hybridize only with sequences downstream of the fifth exon of this gene (A5 alpha). We propose that all these transcripts result from distinctive cut-no-splice events, occurring at different intron-exon borders of OXI3 pre-RNAs depending on the mutational site within the apocytochrome b gene. The box9 mutant M4458 and the box7 mutant M1431 lack detectable 18S mRNA for subunit I of cytochrome c oxidase. The box9 mutants M4751 and M4701 contain reduced amounts of this mRNA. The fact that these loci complement each other (B. Weiss-Brummer, G. Rödel, R.J. Schweyen, and F. Kaudewitz (1982) Cell 29, 527-536), therefore, suggests that mutations within the different functional domains of bI4 lead to different defects in the processing of OXI3 transcripts. This, together with the defects observed in bI2 and bI3 mutants, implies that the box effect (i.e., the interaction between these two split genes) is not mediated by the box7 element alone. The possibility is discussed that mutated apocytochrome b intronic reading frame products lead to these aberrant events in the processing of transcripts of the gene for subunit I of cytochrome c oxidase.