All existing descriptions of nutrient distributions in the Strait of Gibraltar suggest that the Atlantic water brings to the Mediterranean Sea nutrients in the Redfield ratio (N:Si:P = 16:15:1). Here, the N:Si:P molar ratios (¿Standard Error), obtained in April 1998, are used to show that in the Atlantic water at the western entrance of the Strait this ratio is lower (13.8(¿0.5):12.1(¿1.0):1) than the classical Redfield ratio; it is close to the Redfield ratio in the middle of the Strait (15.6(¿0.6):10.7(¿0.9):1), and increases dramatically to 23.6(¿3.4):29.1(¿4.5):1 at the eastern entrance of the Strait. In the Mediterranean water, the N:Si:P ratio has a quite similar trend with 31.5(¿6.0):26.5(¿3.6):1 in the east, 20.4(¿0.2):31.5(¿11.1):1 in the middle and 18.1(¿0.6):17.6(¿0.7):1 in the west of the Strait. The physical and biological processes that account for the observed spatial variability of the N:Si:P ratio along the Strait are identified. We estimated that in the Atlantic water entering the Mediterranean Sea, about 84% of the variability in N:Si:P molar ratio is due to biological and 16% to physical processes.