We present behavioral observations and multilocus DNA fingerprinting data on Semipalmated Plovers (Charadrius semipalmatus) breeding in the sub-Arctic. We predicted that, where a large time/energy investment by males during incubation and chick-rearing is crucial for successful reproduction, both extra-pair copulation and fertilization rates would be low. Extra-pair copulations were infrequent (7% of total copulations), as were within pair copulations (0.44 hr⁻¹). Copulations occurred, on average 6.2 days prior to clutch initiation. Males spent 64% of their time in visual contact with their mates. Mate guarding during the laying period was significantly more pronounced in coastal neighborhoods of breeding birds than among solitary-nesting pairs. Extra-pair fertilizations occurred in 4% of families resulting in an extra-pair paternity rate of 4.7%.