The sperms are required to through the zona pellucida to become capable of contributing to normal fertilization. In fertilized eggs, the primary block to second sperm entry occurs on the egg plasma membrane or zona pellucida depending on species. The microinjection is a powerful tool to directly approach and resolve several important questions related to fertilizing ability of various phenotype of foreign sperm injected into the egg cytoplasm after puncture of the egg plasma membrane. As a control experiment, unfertilized mouse egg was injected with a sperm obtained from cauda epididymis of mouse to determine whether the sperm can develop to decondensation for normal fertilization. A second sperm was then injected into the fertilized egg just prior to the first cleavage division to test whether the second sperm can produce a third pronucleus. The eggs injected with sperm were then fixed and stained for observation of sperm decondensation under the microscope. We report here that a third pronucleus is produced when a mouse sperm was injected into normally fertilized egg of mouse.