The Eastern Mediterranean Sea (EMS) is a relatively small ocean basin with an unusually high nitrate to phosphate ratio in the deep waters (∼28:1). As a consequence, the typical winter phytoplankton bloom is P rather than N limited. Based on a critical review of the existing evidence, we conclude that the unusual nutrient ratio is due to high N:P values in all the external nutrient inputs to the EMS, coupled to low denitrification rates within the ultra-oligotrophic basin. Thus, we rule out the alternative hypothesis that P limitation in the EMS is due to regionally high rates of diazotrophic N2 fixation. The first line of evidence is a basin-wide nutrient budget, which demonstrates that N removal from the EMS is balanced by N inputs by rivers and atmospheric deposition without the need to invoke additional N2 fixation. The budget further indicates that riverine and atmospheric inputs all have N:P ratios that significantly exceed the Redfield ratio (16:1), and that atmospheric deposition is the major external source of bioavailable N to the EMS. The second line of evidence is a series of recent δ15N measurements showing depleted values in both wet and dry N deposition in the EMS. Hence, the depleted δ15N values of nitrate measured in deep waters of the EMS (2.4±0.1‰) do not reflect N2 fixation, but rather a signal inherited from the nitrate deposited from the atmosphere. The few direct rate determinations of N2 fixation provide the third line of evidence: they show very low activities in both pelagic and coastal areas (