Effect of sperm selection according to the degree of motility after insemination on in-vitro penetration was examined by using a new in-vitro fertilization system designated as a climbing-overa-wall (COW) IVF method. When the sperm penetration rate in the COW-IVF method was compared with a standard method at the same sperm concentration (5 X 10s cells/ml), the rates (95.1 ±1.9 and 98.2 ±1.0%, respectively) were similar, but the incidence of monospermic penetration was higher in the COW-IVF (25.5 ±4.5%) than the standard method (10.4 ±2.5%). When sperm concentration was changed from 0.5 × 105 to 10 × 105 cells/ml in the COW-IVF method, sperm penetration rate was higher at a higher concentration, whereas monospermic penetration rate was increased at a lower concentration. The proportion of monospermic oocytes in matured oocytes was similar among sperm concentrations, 0.5 × 105 to 5 × 105 cells/ml, at fertilization in the COW method. These results demonstrate that the COW-IVF method, selection according to the degree of sperm motility after insemination, can increase the normal penetration of frozen-thawed boar spermatozoa into IVM oocytes without any reduction in the sperm penetration rate.