Among birds, males investing highly in paternal care should be under par- ticularly strong selection pressure to prevent extrapair paternity in their broods. Current literature implies that to obtain a high certainty of paternity, males should invest in pater- nity-assurance behaviors, and this makes them more likely to increase their investment in reproduction. The Purple Sandpiper (Calidris maritima) is a relatively long-lived arctic shor- ebird with a socially monogamous pair bond and strong mate and territory fidelity by both sexes. Paternal investment is high because males alone perform all of the brood care from hatching to fledging. We investigated the mating strategies of male and female Purple Sand- pipers through genetic analysis and examined the possibility that this species exhibits pa- ternity-assurance behaviors. We found no evidence that males exhibited behaviors such as frequent copulation or mate guarding during their mate's fertile period. Furthermore, high rates of copulation were not necessary for pairs to achieve complete fertility of the clutch. Multilocus DNA fingerprinting revealed little evidence for extrapair fertilizations: only 1 of 82 offspring was sired by an extrapair male. These results suggest that male and female Purple Sandpipers rarely seek extrapair copulations (EPCs). In contrast to several recent studies with similar findings, we believe that the apparent rarity of EPCs results because females have few genetic or other benefits to gain from seeking them. Thus, paternity-as- surance behaviors by male Purple Sandpipers were not necessary for achieving full paternity of the clutch, and such behaviors cannot be said to have led to the observed high level of paternal investment in reproduction. Uniparental brood care, however, is associated with a high certainty of paternity in this species. Received 3 June 1997, accepted 13 November 1997.