Although cheap compared to eggs, sperm are still costly to produce and may deteriorate if stored 1 and 2. Therefore, selection should favour the ability of males to adjust the quantity or quality of sperm in response to fertilisation opportunities. The males of some species of insect, for instance, may adjust testis size, sperm number or ejaculate composition in response to the social environment [3]. Males of certain rodents, fish and insects increase sperm production in response to female presence 4, 5 and 6. However, males typically have at least some sperm always ready. Here, we show that in the insect-killing nematode Steinernema longicaudum, males that develop alone contain no sperm. Only after several hours with a female, but not another male, sperm are produced so that progeny can be sired.