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Hatcheries support fisheries and aid in the recovery of endangered wild populations. Evidence for reduced reproductive success (RS) in wild-spawning hatchery-reared salmon compared with that in wild-born fish invites questions about the impact on subsequent generations. Immune gene-dependent mate preference is one mechanism known to influence salmonid fitness. We evaluated mate choice and correlates of RS to better understand fitness differences between hatchery-reared and wild-born fish using a previously constructed genetic pedigree of coho salmon ( Onchorhynchus kisutch) from the Umpqua River, Oregon. Two years (2005 and 2006) of three wild-spawning mate pair classes were examined: wild × wild (W × W), hatchery × hatchery (H × H), and wild × hatchery (W × H). We found no evidence for mate choice within mate pair classes based on microsatellites linked to the major histocompatibility complex (MHC) and immune-relevant expressed sequence tags. Greater W × W mate pair RS was associated with increased MHC diversity in 2005 and 2006, while greater W × H mate pair RS was correlated to intermediate MHC diversity in 2006. We found no correlation between MHC diversity and H × H mate pair RS. Our results suggest that greater MHC diversity between wild-born coho pairs may increase offspring survival. [ABSTRACT FROM AUTHOR]