We report on likely mixed paternity in a natural population of snow skinks (Niveoscincus mirolepidotus) from alpine Tasmania, Australia. This species is nonterritorial and males guard females after copulation, suggesting that guarding behavior has evolved to prevent rival mating of still-receptive females. To what degree does this mate-guarding prevent rival copulations? We sampled gravid females at random in the wild and looked for within-clutch mixed paternity among their offspring using amplified fragment length polymorphism (AFLP). Incorporating all visualized fragments, offspring band-sharing based on maternal bands was 0.94 (60.05, SD), whereas for paternal fragments it was 0.54 (60.46, SD). We then tested paternal band-sharing scores for all young of pairs against the mean score of the maternally inherited fragments to assess whether paternal genetic variation was larger than for a known single parent, hence, suggesting multiple sires. To reduce the risk of unequal sampling of polymorphic maternal and paternal fragments, we based our statistical tests on heterozygous bands only. Offspring band sharing based on maternal heterozygous fragments was on average 0.68 (60.22, SD), versus 0.35 (60.33, SD) based on paternally inherited fragments. In six of eight clutches (75%), at least one pair of young in a clutch had paternal scores outside of the confidence interval for a single parent (i.e., the mother). Thus, mixed paternity seems to be widespread in this population, despite prolonged postcopulatory mate-guarding by males.