This experiment was conducted to investigate the effects of breeds, medium supplements and ovarian phases on the developmental pattern of bovine follicular oocytes. Ham`s F10 powder was dissolved in the 3rd demineralized and 5th distilled water and supplemented with 10% heat inactivated fetal bovine serum (HI-FBS) or 0.4% bovine serum albumin (BSA). The medium was equihlmated for 16±2 hours in 5% CO₂ incubator and adjusted in pH 7.4-7,6 and in 280±10 mOsm. The oocytes were recovered from small antral follicles (2 to 5㎜) and matured for 24 to 26 hours at 39℃. Matured oocytes were fertilized with ejaculated sperm (1-2 x 10^6/㎖) and cultured for 4 days at 38.5℃ in 5% CO₂ in air. The cleavage rate of oocytes was higher in Korean native cattle (KNC; 22.0%, 18/82) than ins Holsteins (4.4%, 4/93). Eleven among eighty two (13.2%) of KNC oocytes developed to morula stage, but none of Holstein oocytes cleaved up to 8 cells. Of the developed ova, rate of embryos with regular blastomeres was also higher in KNC (∼65%) than in Holsteins (∼33%). Holstein oocytes were fertilized with KNC sperm to clear the reason why the developmental potential of Holstein oocytes was extremely limited. Their cleavage rate was 16.7%o (4/24) and was higher than that of Holstein oocytes fertilized with Holstein sperm. Therefore the low potential in vitro of Holstein oocytes was due to oocytes and not to sperm. Cleavage rate of KNC oocytes was higher in FBS supplemented medium (22.0%, 18/82) than in BSA supplemented medium (14.8%, 4/27). The oocytesdeveloped to morula stage in FBS-medium, but non-cleaved up to 3 cells in B SA-medium. Of the developed ova, rate of embryos with regular blastomeres is higher in FBS-medium (∼65%) than in BSA-medium (∼33%). Of the uncleaved 1-cell ova, rate of fragmental ova was significantly higher in BSA-medium (33.3%) than in FBS-medium (14.6%). By contrast, the rate of deformed ova was significantly lower in BSA-medium (7.4%) than in FBS-medium (25.0%). The cleavage rate of oocytes was not different between luteal phase and follicular phase, but the rate of embryos with regular blastomeres was higher in luteal phase than in follicular phase. In conclusion, the rates of cleavage and normal development of follicular oocytes were higher in KNC than in Holsteins, in FBS-medium than in BSA-medium, and in luteal phase than in follicular phase. The cleavage rate of oocytes was related to the rate of normal development. Immature-follicular oocytes of KNC can normally mature and develop to morula stage in Ham`s F10 supplemented with 10% FBS but that of Holsteins can not.