Naf1 is an essential protein involved in the maturation of box H/ACA ribonucleoproteins, a group of particles required for ribosome biogenesis, modification of spliceosomal small nuclear RNAs and telomere synthesis. Naf1 participates in the assembly of the RNP at transcription sites and in the nuclear trafficking of the complex. The crystal structure of a domain of yeast Naf1p, Naf1Delta1p, reveals a striking structural homology with the core domain of archaeal Gar1, an essential protein component of the mature RNP; it suggests that Naf1p and Gar1p have a common binding site on the enzymatic protein component of the particle, Cbf5p. We propose that Naf1p is a competitive binder for Cbf5p, which is replaced by Gar1p during maturation of the H/ACA particle. The exchange of Naf1p by Gar1p might be prompted by external factors that alter the oligomerisation state of Naf1p and Gar1p. The structural homology with Gar1 suggests that the function of Naf1 involves preventing non-cognate RNAs from being loaded during transport of the particle by inducing a non-productive conformation of Cbf5.