Fifty red and white Shikabrown cocks were purchased and fed on a diet of layers mash with 18.0% crude protein, 95.6% dry matter, 17.1% crude fibre and 3.0% nitrogen. The experiment took place betwen January and May, 2005. Twentyâ€Â�five of the cocks were infected with 106.0EID50 of a velogenic Kudu strain Newcastle disease virus while twentyâ€Â�five served as control. The cocks had their packed cell volume, total protein, hemagglutination inhibition ND antibody titres (HIT), semen characteristics, testosterone profiles and gonadal sperm reserves and body and organ weights determined weekly for six weeks post infection. At six weeks postâ€Â�infection, twenty control and twenty infected cocks were slaughtered. Their internal organs were examined in situ and later removed and weighed; the dimensions of the testes were also taken. The semen was evaluated for volume and colour; and the latter was graded as: grade 1 = creamy; 2 = milky and 3 = watery. There was no significant difference in the packed cell volume of the control and infected red and white cocks. The values of the total protein did not show any significant difference between the control and infected red and white cocks. The antibody titres of the control red and white Shikabrown cocks were significantly (P<0.05) lower than the antibody titres of the infected red and white cocks. The cloacal temperatures and body weights did not show any significant difference between the red and white cocks. The semen volume of infected red cocks showed a general increase over that of the control red cocks. The semen volume of the control white cocks was significantly higher than that of the infected white cocks. The white ix Shikabrown cocks had higher semen volume than the red Shikabrown cocks. The red cocks had slightly better semen colour than the white cocks. The control white cocks had better spermatozoa motility than the infected white cocks, while the infected red cocks had better spermatozoa motility than the control cocks. The spermatozoa concentration of the control white cocks was consistently better than that of the infected white cocks. The spermatozoa concentration of the infected cocks was lower than that of the control cocks. The white cocks had better spermatozoa concentration than the red cocks. The control white Shikabrown cocks had significantly (P<0.05) higher per cent live spermatozoa than the infected white cocks. The infected red cocks had significantly (P<0.05) lower per cent live spermatozoa than the control red cocks. The control white cocks had significantly (P<0.05) higher per cent live spermatozoa than the control red cocks. The infected red and white Shikabrown cocks had higher percentage total spermatozoa abnormalities than the control red and white cocks. The mean testosterone concentration of the red and white cocks decreased postâ€Â�infection from week 1 to 6. The gonadal sperm reserves of the control red and white cocks were not significantly different but the gonadal sperm reserves of the control white cocks was significantly (P<0.05) higher than the gonadal sperm reserves of the infected red and white cocks. The total gonadal sperm reserves of the control white cocks was significantly (P<0.05) higher than the total gonadal sperm reserves of the infected red and white cocks. The left testicles of the control red and white cocks were significantly (P<0.05) heavier than those of the infected red and white cocks. x The liver, spleen, thymus, heart and adrenal glands of the infected red Shikabrown cocks were heavier than those of the control red Shikabrown, and white cocks and infected white Shikabrown. The weights of the spleen and brain were not signifiantly different in both the control and infected red and white cocks. The bursal weight of the control white cocks was heavier than that of control red, infected red and infected white cocks. The control red cocks had the heaviest adrenal glands. The infected red cocks had the highest liver body weight ratio. The spleen body weight ratio was the same for both control and infected red and white cocks. The control white cocks had the least thymus body weight ratio. The infected red cocks had the highest heart body weight ratio. The brain body weight ratio was the same for both control and infected cocks. The control white cocks had a higher bursa body weight ratio than the control red and infected red and white cocks. The infected red cocks had the highest adrenal body weight ratio. Only one infected white cock showed perivascular infiltration of lymphocytes and foci of glial cells. It can be concluded that challenging the cocks with the velogenic Newcastle disease virus led to the cocks having higher antibody titres which gave them more protection. The infected red Shikabrown cocks had slightly higher antibody titres than the infected white Shikabrown cocks hence they were more protected from the velogenic Newcastle disease virus. The white cocks had better semen quality than the red cocks but more prone to the Newcastle disease virus. In a Newcastle disease endemic environment like Zaria farmers keeping Shikabrown chickens should adequately and routinely vaccinate them against the disease.