A method of inhibiting the self-splicing of a Group I intron is disclosed. The method uses an oligonucleotide having a sequence essentially identical to a guide sequence found in the 5 flanking exon and terminates with a 3 ribonucleoside. Usually the oligonucleotide has N3P5 phosphoramidate or N3P5 thiophiosphoramidate linkages rather than phosphodiester linkages. A method of inhibiting the growth of organisms having Group I intron, particularly certain pathogenic fungi including P. carinii, C. albicans, and A. nidulans using the oglionucleotide is also provided.