A novel in vitro selection method was developed to isolate RNA sequences with coenzyme-synthesizing activities. We used size-heterogeneous libraries containing randomized ribonucleotide sequences of four different lengths (30N, 60N, 100N, and 140N), all with 5'-ATP initiation. Two RNAs, CoES7 (30N) and CoES21 (60N), are able to catalyze the synthesis of three common coenzymes, CoA, NAD, and FAD, from their precursors, 4'-phosphopantetheine, NMN, and FMN, respectively. Both ribozymes require divalent manganese for activities. The results support the availability of these coenzymes in an RNA world, and point to a chemical explanation for the complex bipartite structures of many coenzymes.