An experiment was made in which artificial substrata immersed during 12 weeks at a depth of 45 feet (ca 14 m) below mean sea level, were periodically examined in situ by means of aqualung diving, as well as in the laboratory, in order to study succession of marine fouling organisms. Results may be summarized as follows: 1. A biological sequence was manifested by the following phases in succession: bacteria, diatoms, hydroids, ectocarpales, barnacles and bryozoa. These populations once they settle, are not subsequently eliminated from the system, though their abundance may vary considerably in magnitude. 2. This succession is essentially similar to that produced at the surface with the elimination of a green alga-phase which may be due to insufficient light. 3. The successive phases are independent of the nature of substratum. The role played by the latter may result in lagging in time of settling of these organisms, retarding their growth or affecting their quantity. 4. Biotic succession seems also independent of seasonal variations of the organisms concerned. Generally speaking, the panels nearer to the bottom support less growth than those higher up, which may be due to the scouring effect of the sand at the bottom. Besides, the amount of growth may sensibly differ between the two surfaces of the same panel. 5. The experimental panels may be arranged according to their suitability for growths in a descending order as follows: Wood, plexiglass, vinyl acetate, glass, brass, zinc, stained steel and copper. 6. Interrelations of the successive phases are discussed. Stress is made upon the food relations in the fouling community. 7. The fouling community is considered as a growing mechanism, manifested by successive phases which may ultimately lead to a “climax”. The factors affecting this mechanism are complex and involve both physical and chemical changes in the substratum, as well as complex food relations among the surviving generations. Foreign animals and plants such as crabs, fish, encrusting algae etc,. may find suitable niches in this biotope and consequently add to the complexity of the system.