Bacterio-pathological investigation on 1751 dead chickens during one year period from January to December 2002 at the BRAC Poultry Disease Diagnostic Centre, Gazipur showed that 39.81% (n=697) cases with seven types of different bacteriological diseases of which salmonellosis (n=385), colibacillosis (n=147) and fowl cholera (n=114) were found significantly higher rate of prevalence then staphylococcosis (n=6), gangrenous dermatitis (n=17), necrotic enteritis (n=24) and infectious coryza (n=4). Accordingly, avian salmonellosis, colibacillosis and pasteurellosis were selected for detailed investigation. Age wise prevalence of avian salmonellosis showed highest infection rate in adult layers (53.25%) in comparison to brooding (14.55%), growing (16.10%) and pullet (16.10%) chickens. The avian colibacillosis was found widely prevalent in all age groups of chickens (9.52 to 36.73%) with specially high prevalence rate in adult layer birds (36.73%). Fowl cholera was recorded in chickens more than two weeks of age with significantly (p Salmonella pullorum ), avian colibacillosis ( Escherichia coli) and avian pasteurellosis ( Pasteurella multocida) were characterized by bacteriological methods which were also subjected to pathogenicity study in 52-day old broiler chickens. Pathogenicity study showed that the incubation period of these three bacterial diseases were recorded as 96 hours and clinical signs appeared on 4 th day of inoculation and observed that S. pullorum , E. coli and P. multocida resulted 100% morbidity in chickens. Key words: Characterization; pathogenicity; salmonellosis; colibacillosis; pasteurellosis; chickens doi: 10.3329/bjvm.v2i1.1926 Bangl. J. Vet. Med . (2004). 2(1) : 01-08