Chemiluminescence　was　observed　when　2,6,7-trihydroxy-9-(4＇-chlorophenyl)-3-fluorone　was　oxidized　with　hydrogen　peroxide　in　alkaline　solution　in　the　presence　of　trace　amounts　of　Co-II,　Trace　amounts　of　Co-II　catalysed　this　chemiluminescence　reaction　strongly,　especially　in　the　presence　of　the　cationic　surfactant　cetyltrimethylammonium　bromide,　and　the　chemiluminescence　intensity　was　proportional　to　the　concentration　of　Co-II,　The　optimum　conditions　for　the　chemiluminescence　reaction　were　investigated　in　detail,　A　chemiluminescence　method　for　the　determination　of　ultratrace　amounts　of　Co-II　was　developed　with　a　detection　limit　of　0.07　ng　ml(-1),　The　relative　standard　deviation　was　2.1%　for　6　ng　ml(-1)　Co-II,　The　method　was　used　for　the　analysis　of　water　samples　without　prior　separation.