The study reported in this paper demonstrated that Humboldt penguins at Punta San Juan, Peru, despite forming pair-bonds, are not strictly monogamous in their mating behavior: 19.2% of the study males and 30.7% of the study females (21 nests) engaged in extrapair copulations. The total number of completed matings observed during the course of this study was 106, of which 17.9% were extrapair copulations. Using DNA fingerprinting we demonstrated that none of these extrapair copulations resulted in extrapair fertilizations; all 49 offspring were attributed to the putative father. Location of copulations suggested that females solicited these extrapair copulations because 89.2% of Humboldt penguin within-pair copulations occurred at the home burrow, yet extrapair copulations took place at a different location based on the sex of the penguin. Extrapair copulations by males occurred at their nest, whereas females conducted 92% of their extrapair copulations away from the nest. These results are most consistent with mate-appraisal and epiphenomenal hypotheses.