The present invention provides an aptamer or pool of aptamers (nucleic acid sequences) that bind(s) to a target molecule on the surface, accessible from the surface or inside of a mammalian sperm cell and a method for producing the aptamers. The method comprises contacting a collection of different nucleic acid molecules with the target molecule under conditions favorable for binding between at least one of the nucleic acid molecules and the target molecule, to form at least one complex comprising the nucleic acid molecule bound to the target molecule, wherein each of the nucleic acid molecules comprises at least one segment of randomized nucleotide sequences. The complexes are then separated from the unbound nucleic acid molecules and unbound target molecules, and the bound nucleic acid molecule is recovered from the separated complex. The aptamers are used to separate sperm cells based upon sperm cell qualities, such as whether the cells contain X chromosomes or Y chromosomes.