Petrels of the genus Pterodroma are among the most enigmatic of seabirds, often nesting on remote islands, ranging far from land, and posing identification challenges. The Hawaiian and Galapagos populations of the Dark-rumped Petrel (Pterodroma phaeopygia) were recently elevated to species rank by the AOUP (2002) as the Hawaiian Petrel (P. sandwichensis) and the Galapagos Petrel (P. phaeopygia), respectively (we use the term “Dark-rumped Petrel” hereafter to refer to the species pair). These taxa appear very similar, and some authors have considered them “doubtfully distinct” at even the subspecies level (Jouanin and Mougin 1979). The Galapagos Petrel averages larger in linear dimensions, whereas the Hawaiian Petrel averages heavier (Simons and Hodges 1998, Tomkins and Milne 1991). There is considerable overlap in all standard measurements, however, and no single variable is sufficient to support the recognition of the two populations as subspecies by the standard 75% rule when the formula presented by Patten and Unitt (2002) is applied to the measurements tabulated by Tomkins and Milne (1991). Genetic and vocal differences have also been reported (Browne et al. 1997, Tomkins and Milne 1991). Here we offer preliminary suggestions for distinguishing these taxa at sea. We believe the characters we describe here will enable the specific identification of birds seen reasonably well, and we encourage observers to test and refine our criteria. We were unable to obtain good photos from Hawaiian waters and have instead used photos from California, which show the characters of the (presumed) Hawaiian Petrels we have seen around Hawaii. The upper photo on this issue’s back cover of a presumed Galapagos Petrel was taken by George Armistead on 21 July 2005 near Isla San Cristobal, Galápagos; the lower photo of a presumed Hawaiian Petrel was taken by Malcolm and Michael Boswell off Fort Bragg, Mendocino County, California, on 13 August 2006.