The 'RNA world' hypothesis, which assumes that the chemical processes that led to the appearance of life were carried out by RNA molecules, has stimulated interest in catalytic reactions involving oligonucleotides such as catalytic RNA (ribozymes). Naturally occurring ribozymes have, for example, been shown to efficiently catalyse the formation and cleavage of nucleic-acid phosphodiester bonds, and this narrow range of RNA-catalysed reactions has been subsequently expanded by in vitro selection methods to include ester and amide bond formation S(N)2 reactions and porphyrin metallations. Carbon-carbon bond formation and the creation of asymmetric centres are both of great importance biochemically, but have not yet been accomplished by RNA catalysis. A widely used reaction that creates two new carbon-carbon bonds and up to four stereo-centres is the Diels-Alder cycloaddition, which occurs between a 1,3-butadiene and an alkene. Here we report the successful application of in vitro selection to isolate pyridine-modified RNA molecules that catalyse a Diels-Alder cycloaddition. We find that the RNA molecules accelerate the reaction rate by a factor of up to 800 relative to the uncatalysed reaction.