Marbled Murrelets (Brachyramphus marmoratus) observed at sea usually are in pairs throughout the year. Although it has been assumed that these pairs are mates, this assumption has not been formally examined. Using data from three study sites during the breeding seasons of 1997–2001, we found that 92% of the birds that were paired at capture were of male-female pairs, and that paired females were more likely (73%) to be producing eggs than were single females (8%). Fourteen of fifteen pairs were tracked to a single nest location per pair. No pair members caught at sea were found breeding at separate nest sites. One pair was caught in two successive seasons, suggesting that at least some pairs are long lasting. Notably, pair members breeding together and radio tracked throughout the summer were detected without their breeding partners for 77% of the time. Thus, while pairs of Marbled Murrelets observed at sea most likely are members of a breeding pair, single murrelets observed at sea should not be assumed to be unpaired or nonbreeders.