Abstract: In order to study the structural and functional organization of the eukaryotic nucleolus, we have started to isolate and characterize nucleolar components of the yeast Saccharomyces cerevisiae. We have identified a major 38 kd nucleolar protein (NOP1), which is located within nucleolar structures resembling the dense fibrillar region of mammalian nucleoli. This 38 kd protein is conserved in evolution since affinity-purified antibodies against the yeast protein stain the nucleolus of mammalian cells in indirect immunofluorescence microscopy and the yeast protein is decorated by antibodies directed against human fibrillarin. Affinity-purified antibodies against the yeast NOP1 efficiently precipitate at least seven small nuclear RNAs involved in rRNA maturation. We have cloned the gene encoding the yeast NOP1 protein. Haploid cells carrying a disrupted copy of the gene are not viable, showing that NOP1 is essential for cell growth. The gene codes for a 34.5 kd protein which contains glycine/arginine rich sequence repeats at the amino terminus similar to those found in other nucleolar proteins. This suggests that NOP1 is in association with small nucleolar RNAs, required for rRNA processing and likely to be the homologue of the mammalian fibrillarin.