The testis is an extremely heterogeneous organ, containing numerous compartments types. Morphometric studies were performed of 3 avian species (pigeon, pheasant and chicken) to determine volume density absolute volume, numerical density, total number of serminiferous tubule components, and sperm production, especially those related to the Sertoli cell, and to make comparisons among the species. Volume density of seminiferous tubule components per testis was determined by point counting method. Testis volume and sperm production were measured by routine techniques. Numerical density (the number of cells per unit volume of testis) of seminiferous tubule components per testis was determined by morphometry (Floderus method). The volume density of seminiferous tubules per testis was 91.58, 92.18 and 94.21% in pigeon, pheasant, and chicken, respectively. The volume density of spermatogonium, spermatocyte, spermatid, spermatozoon, and Sertoli cell did not produce significant changes in the three species. The absolute volume of spermatogonium, spermatocyte, spermatid, and Sertoli cell showed significant changes in the three species (p<0.05). The average volume of Sertoli cell ranged from 758.34(pheasant) to 1,212.9 ㎛$^3$(chicken) and was not significantoy different in the three species(p>0.05). The number of Sertoli cells per testis showed significant differences in the three species : 34.52 $ imes$10(sup)6, 186.82$ imes$10(sup)6, 810.62$ imes$10(sup)6 in pigeon, pheasant, and chicken, respectively(p<0.05). The sperm production was significantly different in the three species : 3,018$ imes$10(sup)6, 993.9$ imes$10(sup)6, and 8.9$ imes$10(sup)6 in chicken, pheasant, and pigeon, respectively(p<0.05). These results suggest that number of Sertoli cells may be more important than Sertoli cell size in explaining the difference in sperm production among the three species.