In teleost fish as in other vertebrates two basic functions representing respectively the humoral and cellular aspects of immunity are mainly involved in innate defences opposed to bacterial infections. Bacteriostatic or bactericidal activity of normal serum may result from the action of different effectors among which the complement system usually plays the major part. Phagocytic activity is a characteristic of specialized lymphoid cells which have developed strong capacities in motility, chemosensitivity, capture and killing of bacteria It occurs at some degree in the humoral compartment and more generally at tissular level, in peripheral tissues or lymphoid organs. In salmonids infected with the cold-water flavobacteriosis agent, Flavobacterium psychrophilum, few researchers have considered these functions yet and a lack of concordance among their observations results from the diversity of the fish species, developmental stages and experimental methods used in these studies. This represents a serious difficulty for drawing a global representation of the fish and bacterium interrelationships in the course of flavobacteriosis. We have established a collection of isogenic homozygous lines of rainbow trout (RT) using a combination of mitotic and meiotic steps of gynogenesis [1]. These fish were repeatedly tested with virulent strains of F. psychrophilum, and lines consistently exhibiting a range of susceptibility or resistance to the bacterium were identified [2]. Such lines represent a convenient material for comparative study of the effectiveness of innate or specific antibacterial defences in susceptible and resistant fish. Here are reported recent observations about normal serum bactericidal effect (NSB) and phagocytosis obtained using several of these lines.