Riemerella anatipestifer (R. anatipestifer) infection in ducks was studied in Assiut and El-Menia Governorates since year 2002-2004. Clinical picture of the infection was reported in ducklings aged 1-8 weeks old as occulonasal discharges, poor growth, anorexia while nervous manifestation and ataxia. In older ages (10-18 weeks), signs noticed as sinusitis, poor growth and incoordination. Bacteriological examination of clinically diseased ducklings revealed 10-12% positive cases, while in older ages the percentage of positive cases was lower (3.3-6%). At necropsy, lesions showed variable degrees of serositis (pericarditis, perihepatitis, and airsacculitis), and swollen joints. Serotyping using agar gel precipitation test (AGPT) revealed isolation of serotypes 2 and 5. Serotype 2 represented 34.69% and 43.75% of positive cultures isolated from ducklings and older age respectively. Serotype 5 represented 14.28% and 9.37% of positive cultures isolated from ducklings and older age respectively. Untypable strains represented 51.0% and 46.87% of positive cultures isolated from ducklings and older age respectively. On experimental infection via the intramuscular (I/M) route, 100% morbidity and, 90% mortality rate by day 7 post inoculation were recorded. Two birds died peracutely with septicemia within 24 hours post inoculation. In case of intranasal (I/N) route of infection, 80% morbidity and 20% mortality rates were recorded by day 7 post inoculation. Oral route challenge displayed lower morbidity and mortality rates (40% and 10%). The challenged birds showed clinical picture and necropsy lesions similar to natural infection with exception of sinusitis after 48 hours post inoculation. Peracutely dead birds showed progressed hemorrhages on the heart, liver, spleen and lung. Minimum inhibitory concentration (MIC) of tested antimicrobials showed susceptibility of R. anatipestifer isolates to penicillin, amoxicillin, enrofloxacin, lincospectin (lincomycin-spectinomycin), oxytetracycline, and cephalosporin. Complete resistance to aminoglycosides (streptomycin,gentamicin) and sulfadimethoxine was demonstrated. Plasmid profile analysis of antimicrobial resistant isolates showed high rate of plasmid acquisition. Similar plasmid was detected in isolates of serotype 2 with molecular weight (MW) of 4 Megadaltons (MDa). Plasmids of MW 2.4 and 3.5 MDa were detected in two isolates of serotype 5. Only one strain of tested untypable isolates possessed two plasmids with high MW of 9 and 10.4 MDa.