The eukaryotic translation initiation factor 1 binds to the ribosome during translation initiation. It is instrumental for initiator-tRNA and mRNA binding, and has a function in selection of the authentic start codon. Here, we show that the archaeal homolog aIF1 has analogous functions. The aIF1 protein of the archaeon Sulfolobus solfataricus is bound to the small ribosomal subunit during translation initiation and accelerates binding of initiator-tRNA and mRNA to the ribosome. Accordingly, aIF1 stimulated translation of an mRNA in a S. solfataricus in vitro translation system. Moreover, this study suggested that the C terminus of the factor is of relevance for its function.