Arrhenotoky (from Greek -τόκος -tókos "birth of -" + ἄρρην árrhēn "male person"), also known as arrhenotokous parthenogenesis, is a form of parthenogenesis in which unfertilized eggs develop into males. The set of processes included under the term arrhenotoky depends on the author. Arrhenotoky may be restricted to the production of males that are haploid, or include diploid males that permanently inactivate one set of chromosomes (parahaploidy) or be used to cover all cases of males being produced by parthenogenesis (e.g. aphids where the males are XO diploids). However, when both males and females develop from unfertilized eggs, the term "deuterotoky" is usually used, instead.
Arrhenotoky occurs in members of the insect order Hymenoptera (bees, ants, and wasps), and the Thysanoptera (thrips). The system also occurs sporadically in some spider mites, Hemiptera, Coleoptera (bark beetles), and rotifers.
In the most commonly used sense of the term, arrhenotoky is used synonymously with haploid arrhenotoky: the production of haploid males from unfertilized eggs in insects having a haplodiploid sex-determination system. Males are produced parthenogenetically, while diploid females are usually produced biparentally from fertilized eggs. In a similar phenomenon, parthenogenetic diploid eggs develop into males by converting one set of their chromosomes to heterochromatin and inactivating them. This is referred to as diploid arrhenotoky.