The U16 and U18 snoRNAs are encoded in introns of the X.laevis L1 ribosomal protein gene and originate from processing of the pre-mRNA. These snoRNAs are newly synthesized around gastrula stage and progressively accumulate during embryogenesis. We show that the basic factors participating in U16 biosynthesis, such as the endonuclease involved in the cleavage reaction and the factors necessary for stabilization of mature snoRNA are present from very early stages. The use of anucleolate mutants has indicated that the synthesis and accumulation of U16 and U18 snoRNAs is not affected in the absence of ongoing rRNA transcription.