Abstract: Due to the hostile climatic environment of the humid tropics characterized by high ambient temperature and humidity all year round, this study was initiated to evaluate the effect of breed and season on the testicular histometric and cellular populations of the domestic fowl. Testes obtained from forty sexually matured cocks consisting of 20 barred Plymouth Rock (bPR) and 20 non-descript Nigerian Indigenous Breeds (NIB) were processed by histological and histometric methods. Volumetric proportions of testicular germ cells remained unaffected by breed or season. Seminiferous tubules occupied 83-87% of the testicular mass in NIB and bPR, respectively. Nuclear diameter of the germ cells and their volumes remained unaffected by breed and season. The mean seminiferous tubule diameter (248.56±12.56 vs. 212.93±9.39 μm) and the seminiferous tubule length/paired testes (221.13±12.59 vs. 133.73±4.52 m) were significantly (p<0.01) influenced by breed with the bPR having the higher values, the mean seminiferous tubule length/g of testicular parenchyma remained unaffected by breed or season. Sertoli Cell Index showed a linear relationship with the testes weight characterized by the equation Y = 0.163x + 1.733, r = 0.9000, R2 = 0.81 (p<0.001). Similar significant (p<0.001) relationships occurred between seminiferous tubule length (r = 0.8651 (p<0.001) ,seminiferous tubule diameter (r = 0.7658, p<0.001), volume % round spermatids (r = 0.9904, p<0.001), testicular parenchyma volume (r = 0.9073, p<0.001) and volume % of seminiferous tubule(r = 0.5474, p<0.001). These parameters can be predicted from testicular weights and testicular histometry provides a reliable tool for the assessment of the reproductive state and potential sperm production capacity in the domestic fowl.