In summer the maximum concentration of dissolved oxygen is nearly always found at a depth corresponding to the lower part of the winter mixed layer. This subsurface maximum is widespread and in some areas may exceed the surface concentration by an increment of more than one milliliter per liter. It is shown that at and above the depth of the summer maximum the oxygen concentration remains very near the saturation concentration throughout the year, and hence depends principally upon the temperature. Seasonal variations in the thermal structure are shown to account for the formation of the subsurface maximum in summer.