Engineered ribozymes are powerful tools for manipulating gene expression in living cells. However, identification of active ribozymes in mammalian cells has relied on empirical assays of a limited number of arbitrarily chosen ribozymes. Here, we synthesized 376 natural and 2625 synthetic variants of pistol ribozymes, and screened for active variants directly in mammalian cells, greatly expanding the ribozyme toolbox for biological applications.