A Vibrio strain Ls001, originally isolated from a body surface lesion of a moribund black porgy (Acanthopagrus schlegeli) in 1994 in Taiwan, was identified as Vibrio vulnificus. The extracellular products (ECP) of the strain were lethal to the fish, and its effects on fish serum in vitro and in vivo are described in the present study. Nine major precipitation arcs were visualized in normal fish serum in a crossed immunoelectrophoresis (CIE) gel using rabbit antiserum to the fish normal serum and staining with Coomassie brilliant blue. Only four and six of the nine major arcs could be tentatively identified by CIE following in vivo and in vitro ECP treatment, respectively. The same two major arcs were both missing following either in vivo or in vitro treatment with ECP. These complex events may significantly contribute to the pathogenesis of V. vulnificus in A. schlegeli.