Surface characteristics of the bovine cumulus-oocyte complex (COC) during maturation in vivo versus in vitro were compared by scanning electron microscopy (SEM). Oocyte diameter changes during maturation were also analyzed prior to oocyte fixation. In vivo matured oocytes were retrieved from live superovulated cows by transvaginal ultrasound-guided aspiration of all visible ovarian follicles (>2 mm in diameter) 0, 12 and 24 h (n=85, 38 and 39, respectively) after hCG administration. In vitro matured (IVM) oocytes refer to those recovered from small antral follicles (2-6 mm in diameter) of slaughterhouse ovaries and then cultured in standard maturation conditions for 0 h (n=188), 12 h (n=138) or 24 h (n=228). SEM analysis showed that immature oocytes of both origins manifested compact COCs with smooth cumulus surface and minimal intercellular spaces among the cumulus cell mass. At 24 h of maturation, more IVM oocytes manifested cumulus expansion compared to in vivo matured oocytes (100%, n=228 vs 44%, n=39), but cumulus expansion was found much more prominently in the cumulus-expanded in vivo matured oocytes than in the IVM oocytes. The zona pellucida showed a fibrous, open mesh-like structure, irrespective of the maturation stage and the source of oocytes. The vitelline surface of the immature oocytes was characterized by distribution of large cellular tongue-shaped protrusions (TSPs) varying in density. These TSPs structures gradually changed to microvilli (MV)-predominant structures upon maturation of the oocytes. At 12 h of maturation, significantly more oocytes (p<0.05) manifested cumulus expansion and transition from TSPs-to MV-predominant vitelline surface structures in the in vitro group (100%, n=106) compared to only 11% (n=38) in the in vivo group. Oocyte diameter decreased gradually in the oocytes maturing in vitro between 0 and 24 h of incubation, being 127 ± 1, 122 ± 1 and 116 ± 1 μm at 0, 12 and 24 h of incubation, respectively. The corresponding values for the oocytes maturing in vivo were 121 ± 2, 129 ± 2, and 101 ± 1 μm, respectively. In conclusion, we found in our study conditions that initiation of oocyte maturation in vivo after hCG administration seemed to be slower than in vitro after maturation culture. But, at 24 h after the onset of maturation more prominent and complete cumulus expansion and more dramatic volumetric changes were observed in the in vivo matured oocytes than in the IVM oocytes.