TypeII restriction and modification enzymes are ideal model systems for studying the specific DNA-protein interaction because of their small size, simple catalytic requirements for activity and recognition sequence specificity. A new restriction endonuclease, XciI, has been partially purified from Xanthomonas citri IFO 3835, it catalytic properties and recognition sequence have been identified. This enzyme XciI cleaves bacteriophage $\lambda$DNA at two sites, pUC9 and pBR322 plasmid DNA at one site, but it cleaves neither $\phi$X174 DNA nor SV40 DNA. The enzyme shows maximum activity at pH values between 7.5 and 9.0, and in the presence of 20 mM MgCl$_2$. The addition of large amounts of Sodium Chloride ($>100mM$) causes a decrease in XciI activity. Temperature for optimum endonuclease activity is $37\,^\circ\!C$. But, Bovine Serum Albumin and sulfhydryl compounds have no influences on XciI restriction endonuclease activity. This restriction endonuclease XciI recognizes the sequence 5``-GTCGAC-3`` 3``-CAGCTG-5`` and cuts at the sites indicated by the arrows, identical with that cleaved by endonuclease SaII (of Streptomyces albus G).