The nucleoli of vertebrate cells contain a number of small RNAs that are generated by the processing of intron fragments of protein-coding gene transcripts. The host gene (UHG) for intron-encoded human U22 is unusual in that it specifies a polyadenylated but apparently noncoding RNA. Depletion of U22 from Xenopus oocytes by oligonucleotide-directed ribonuclease H targeting prevented the processing of 18S ribosomal RNA (rRNA) at both ends. The appearance of 18S rRNA was restored by injection of in vitro-synthesized U22 RNA. These results identify a cellular function for an intronencoded small RNA.