Dioecious (also known as gonochorous) species are those which have two separate sexes: male and female; with individuals belonging to either one sex or the other. The female produces macrogametes (egg cells) while the male produces microgametes (sperm cells), and the union of the two kinds of gametes is what enables reproduction to take place.

Species in which individuals can produce both male and female gametes are called hermaphrodites, and usually still require sex between two individuals in order to reproduce, although sometimes they might be able to self-impregnate. Species which have no sex at all are referred to as asexual.

Note: Only add species to this category if male and female members have been observed or specifically mentioned. Species only generally assumed by the audience to be dioecious should not be added. Be careful to consider the possibility of sequential hermaphroditism, in which individuals start life as males and spontaneously turn female when they grow older; or vice-versa. Therefore, a species of sequential hermaphrodites will appear like a dioecious species at first glance, even though they're not; but a keen observer may notice that the children and/or the elders of such a species will be all of the same sex.