In externally fertilizing animals in which females mate with multiple males at the same time (simultaneous polyandry), the possibility that females accrue genetic benefits that improve offspring viability remains largely unexamined. Here, we investigate whether simultaneous polyandry influences offspring fitness in a wild population of the Grey Foam Nest Treefrog Chiromantis xerampelina. Simultaneous polyandry in this frog is the most extreme reported for any vertebrate, with more than 90% of females mating with 10 or more males during the deposition of a single clutch. We compared growth (using age and size at metamorphosis as proxies) and survival of offspring produced by females that naturally mated with either 1 male (monandrous females) or 10-12 males (polyandrous females). Polyandry did not influence size or age at metamorphosis, but we found that offspring from polyandrous matings had both significantly higher mean survival and reduced variance in offspring survival. These findings implicate a genetic mechanism, but targeted cross-classified breeding experiments that control for both maternal and material effects will be required to conclusively determine whether elevated offspring viability is linked to enhanced genetic diversity, intrinsic sire effects, or genetic compatibility. Irrespective of the causation, the findings provide the first evidence that naturally formed polyandrous matings have increased offspring viability in an anuran amphibian. © 2011 The Author.