XorI and XorII, two sequence-specific endonucleases, have been partially purified from Xanthomas oryzae. XorI and XorII were shown to be isoschizomers of PstI and PuvI, respectively. X. oryzae is a particularly good source of this PvuI isoschizomer because of the high yield of XorII, its simple purification scheme, and its relative stability. Furthermore, XorII was shown to cleave at different positions in its recognition sequence than do at least two of its known isochizomers; XorII cleaves between the C and the G at the 3'-end of its palindromic recognition sequence, 5'-CGATC G-3'. There is a single XorII site in each of the plasmid-cloning vehicles pBR313 and pBR322. Two unusual aspects of XorII digestion are discussed, namely, the kinetics of digestion of pBR313 and pBR322 and the resistance of human DNA to XorII.