The 'RNA world' hypothesis proposes an early stage in the evolution of life in which both genomic and catalytic functions were fulfilled by RNA. The evolution of RNA-catalysed protein synthesis would have been a necessary step in the transition from such an RNA world to modern protein-dominated biology. For this to have been possible, RNA must be capable of catalysing amide-bond formation using acylated carrier RNA substrates as amino-acid donors. We have used in vitro selection and evolution to isolate ribozymes with acyl transferase activity from a pool of random RNA sequences. One of these acyl transferases with a 5'-amino group transfers an amino acid to itself in a reaction that we propose to be analogous to peptidyl transfer on the ribosome.