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　CoII.　Trace　amounts　of　CoII　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　CoII.　The　optimum　conditions　for　the　chemiluminescence　reaction　were　investigated　in　detail.　A　chemiluminescence　method　for　the　determination　of　ultratrace　amounts　of　CoII　was　developed　with　a　detection　limit　of　0.07　ng　ml-1.　The　relative　standard　deviation　was　2.1%　for　6　ng　ml-1　CoII.　The　method　was　used　for　the　analysis　of　water　samples　without　prior　separation.