A method for preparing spermatozoa having an alien DNA of a mature male nonhuman vertebrate transduced thereinto which comprises injecting the alien DNA carried by a liposome for transduction into the testes of the animal thrice or more repeatedly and thus producing spermatozoa having the alien DNA transduced thereinto in the testes of the animal. A transgenic nonhuman vertebrate can be conveniently and efficiently produced by fertilizing an unfertilized egg with such a spermatozoon by natural crossing, artificial fertilization or external fertilization.
