A short term study was made on the ecology and the net productivity of sublittoral populations of benthic macrophytes in, and near, the Ria de Aldan on the Atlantic coast of Spain. A preliminary survey was made using SCUBA diving techniques and this revealed that the most extensive sublittoral communities were those dominated by Laminaria ochroleuca and/or Saccorhiza polyschidee. The effect of depth and the amount and type of water movement on the distribution and productivity of these two populations was studied at 7 localities. There was found to be a narrow zone of transition between the two populations and the depth of this zone was governed by the amount of water movement. The factors which might control the dynamic equilibrium between these two populations are discussed. The total standing crop and productivity per unit area was found to undergo a decrease with both an increase in depth and wave action whilst the highest values of all were in two localities where there was considerable current surge. The maximum productivity of L. ochroleuca (17 metric tons/hectare/a~num) was found in the most sheltered locality studied, whilst the highest value for S. polyschides (39 metric tons/hectare/annum) was found where the current surge was greatest but wave action was only moderate. This maximum value for S. loolyschides is the highest yet reported for naturally occurring beds of submerged seaweeds. Figures for the net efficiency of energy fixation are presented and discussed.