Synopsis.FRESHLY gathered marine diatoms when placed upon an agar medium ejected their protoplasmic masses from the siliceous frustules. The naked cells, enclosed within an intact perizonium, continued to live upon a solid medium for more than nine months, during which they lost their normal shape and power of locomotion. When the cells were returned to an aqueous medium, either static or flowing, they became rehabilitated, re‐establishing their naviculoid form and regaining their power of movement.This work was undertaken at the Admiralty Central Metallurgical Laboratory, Emsworth, and forms part of a general investigation into the r81e of “first colonist” organisms on surfaces immersed in seawater, in connection with the antifouling problem of ships'