Complement is a humoral factor of innate immunity and plays important roles in immune surveillance and clearance of invading pathogens. This system is directly and immediately activated in an immune-competent host, and the altemative（ antibodyindependent）, Lectin （triggered by mannose-binding lectin） and classical pathways （antibody-dependent） have been described in fish. All three pathways converge to the lytic pathway, leading to opsonization or direct killing of the microorganism. The alternative and lectin pathways are ancient mechanisms present in invertebrates as well as ancient vertebrates. Some of the agents, like zymosan and LPS （lipopolysaccharide）, are known activators of the alternative pathway of fish complement. The lectin pathway, which has not been much studied in-teleosts, is activated by the binding of mannose on the surface of microorganisms to a complex of mannosebinding lectin （MBL） and MBL-associated serine protease （MASP）. Fish complements are comparable to those of mammals but some important differences have been described. Fish sertun shows hemolytic activity indistinguishable from that of mammalian compleraent. In general the complement system of the altemative pathway is more active in fish than in mammals, is not heat labile and therefore difficult to preserve. Fish complement has lower optimum reaction temperature than its mammalian counterpart, and its components, like C3. The complement system is composed of numerous proteins and all pathways generate factor C3, which has been described and isolated from teleost species. Complement C3 is a central molecule in the complement system whose activation is essential for all the important functions performed by this system. Complement proteins in fish, like C3 and B factor, are more polymorphic. Here we describe some characteristics of the complement system of fish.