RNA guanylyltransferase, or capping enzyme (E.C. 2.7.7.50) catalyzes the transfer of GMP from GTP to diphosphate-terminated RNA to form the cap structure GpppN. Chlorella virus capping enzyme expressed in E. coli has been purified, treated with GTP and crystallized. X-ray diffraction data have been collected from these crystals as well as for a mercury derivative obtained by soaking the crystals in thimerosal. Selenomethionine RNA guanylyltransferase was purified and crystallized in a similar fashion. The space group is C2221 and the cell parameters are a = 93.3, b = 214.9, c = 105.8 A. Two Hg atoms and two subsets of Se atoms have been localized using difference Patterson and Fourier methods, suggesting that there are two molecules per asymmetric unit.