The Tetrahymena group I ribozyme catalyzes a sequence-specific phosphodiester cleavage reaction on an external RNA oligonucleotide substrate in the presence of a divalent metal cation cofactor¹. This reaction proceeds readily with either Mg²⁺ or Mn²⁺ ion, but when other divalent cations are used as the sole cofactor, no detectable reaction has been reported ^2-5. Cations such as Ca²⁺, Sr²⁺, and Ba²⁺ can stabilize the correct folded conformation of the ribozyme, thereby partially alleviating the Mg²⁺ or Mn²⁺ requirement². However, catalysis by the ribozyme involves coordination of either Mg²⁺ or Mn²⁺ at the active site, resulting in an overall requirement for one of these two cations⁵. Through an in vitro evolution process ^6,7, we have obtained variants of the Tetrahymena ribozyme that are capable of cleaving an RNA substrate in reaction mixtures containing Ca²⁺ as the divalent cation. These findings expand the range of chemical environments available to RNA enzymes and exemplify the power of in vitro evolution in generating macromolecular catalysts with desired properties.