The initiation of translation of picornaviral RNAs takes place by an unusual mechanism whereby ribosomes bind directly to an internal site rather than scan the RNA from the 5'-end. This internal entry mechanism requires a 450-nucleotide segment of the picornavirus 5'-untranslated region. The ribosome binds initially to a site at the 3'-end of this segment, and then may scan the RNA to reach the authentic initiation site. This novel mechanism may be of relevance to the translation of some cellular mRNAs.