Nucleolin, a major RNA binding protein of the nucleolus is found associated mainly to the pre-ribosomal particles and is absent from the cytoplasmic mature ribosomes. The role of this protein in ribosome biogenesis remains largely unknown, and is likely to be reflected by its RNA binding properties. Nucleolin contains in its central domain four RNA recognition motifs (RRM, also called RBD for RNA binding domain) which are conserved among different species. RNA binding studies have revealed that nucleolin interacts specifically with a short stem loop structure called NRE (nucleolin recognition element). We show that nucleolin extracted from human, hamster and mouse cells interacts with the same specificity and affinity to a mouse 5'ETS (external transcribed spacer) RNA fragment which contains a NRE motif. A similar structure within the human 5'ETS is also efficiently recognized by mouse nucleolin. We identified putative NRE not only in the 5'ETS but also in the 3'ETS, ITS (internal transcribed spacer) and in the 18S and 28S RNA sequences. This is in agreement with in vivo cross-linking data and a previous immunocytological analysis of ribosomal transcription units. Interestingly, we found that all the NRE localized in the 28S region are within the variable domains. Despite considerable sequence divergence of these domains, several of the NRE have sequences perfectly conserved between these two species. This suggests that these nucleolin binding sites might be functionally important, in particular for ribosome biogenesis.