These experiments were carried out to produce monozygotic twin by bisection of mouse morula. The morula was bisected by microglass needle without any aid of microinstrument or by lateral incision using sharp blade attached to micromanipulator. Bisected dermi-embryos were cultured and transferred to pseudopregnant recipient mouse. The results obtained in these experiments were summarized as follows: 1. Of 285 decompacted morulae bisected by microglass needle, 180 embryos were divided into two demi-embryos, respectively, without any visible damage. 2. Total 180 pairs of demi-embryos separated from decompacted morula and subjected to in vitro culture were resulted in 85(47.2%) pairs of eu-blastocyst, 40(22.2%) pairs of eu-blastocyst and pseudo-blastocyst, 35(19.4%) pairs of pseudo-blastocyst and 20(11.1%) pairs of trophectodermal vesicle and degeneration. 3. Of 255 intact morulae bisected by microblade, 100(39.2%) embryos were divided into two demi-embryos, respectively, without damage. However, 95(37.3%) embryos were bisected into one normal and one damaged demi-embryos, respectively. The percentage of demi-embryos developed to blastocyst after in vitro culture with and without zona pellucida were 71.4 and 64.9% respectively. 4. Total 15 twins were produced following transfer of the 38 pairs of eu-blastocysts developed from demi-embryos to 25 recipients. However, none of pseudo-blastocyst gave birth to young, 5. The percentage of eu-blastocyst cultured with or without zona pellucida and developed to live young following transfer were 11.1 and 9.3% respectively.