Haploids and double haploids are important resources for studying recessive traits and have large impacts on crop breeding¹, but natural haploids are rare in animals. Mammalian haploids are restricted to germline cells and are occasionally found in tumours with massive chromosome loss^{2, 3}. Recent success in establishing haploid embryonic stem (ES) cells in medaka fish⁴ and mice^{5, 6} raised the possibility of using engineered mammalian haploid cells in genetic studies. However, the availability and functional characterization of mammalian haploid ES cells are still limited. Here we show that mouse androgenetic haploid ES (ahES) cell lines can be established by transferring sperm into an enucleated oocyte. The ahES cells maintain haploidy and stable growth over 30？passages, express pluripotent markers, possess the ability to differentiate into all three germ layers in vitro and in vivo, and contribute to germlines of chimaeras when injected into blastocysts. Although epigenetically distinct from sperm cells, the ahES cells can produce viable and fertile progenies after intracytoplasmic injection into mature oocytes. The oocyte-injection procedure can also produce viable transgenic mice from genetically engineered ahES cells. Our findings show the developmental pluripotency of androgenentic haploids and provide a new tool to quickly produce genetic models for recessive traits. They may also shed new light on assisted reproduction.