The developmental competence of in vitro matured ovine oocytes, cytoplasmically injected with single male or female chromosome-bearing sperm, was investigated. Eighty-five unsorted, 92 ‘female-sorted’ and 74 ‘male-sorted’ ram sperm were injected into in vitro matured sheep oocytes and, two to four hours later, placed into the oviducts of 28 oestrous sheep. The sperm were separated according to sex by analysis of their DNA content with a flow cytometer. One pregnancy was diagnosed by ultrasound after 55 days and a 3 kg male lamb was born after 150 days gestation. This lamb was derived from an oocyte injected with ‘male-sorted’ sperm.