In most species the offspring of closely related parents have reduced fitness compared with the offspring of unrelated parents, a phenomenon known as inbreeding depression. However if parents are very distantly related, their offspring may also have reduced fitness. This pattern, outbreeding depression, has been most commonly observed in plants and only rarely in animals. Here we examine the consequences of inbreeding and outbreeding on juvenile survival of reintroduced Arabian oryx (Oryx leucoryx) in Oman, a population with a small number of founders drawn from a number of sources. Using microsatellite-based measures of inbreeding and outbreeding, there was no apparent relationship between inbreeding or outbreeding and survival when inbreeding and outbreeding were tested in separate statistical models. However when inbreeding and outbreeding were tested in the same statistical model, we found simultaneous inbreeding depression and outbreeding depression acting on juvenile survival. Outbreeding depression may be more common in vertebrates than previously supposed, and conservation strategies that seek to maximize the genetic diversity of managed populations may risk mixing lineages that are sufficiently differentiated to cause outbreeding depression among descendants.