Methods are described for the collection, treatment and uniform discharge of large quantities of gametes, and a measure of fertilization control in the monoecious brown alga, Fucus distichus. Fertilization was examined using the techniques of blister-formation, gamete separation experiments, and electron microscopy. Sperm enter the freshly-discharged egg packets through a mesochite pore and juxtaposition with the eggs early. However, experimental and thinsection data indicate that fertilization does not occur until the eggs dissociate from the mesochite and round up. Hence, the egg surface appears to undergo three functional changes following its release from the thallus: (1) a pre-dissociation state which inhibits fertilization within the mesochite; (2) a dissociation state when fusion of gametes is possible; and (3) a post-fertilization state characterized by the formation of extraneous coats. As the egg is activated by the sperm a nonmembranous layer appears to detach from the egg surface to form an activation layer. This is augmented by fibrous units to function as a fertilization barrier which ultimately thickens to form the cell wall. The area between the eggs within the egg packet is characterized by the presence of fibrous and particulate substances which are continuously given off through the egg surface. These apparently play a functional role in the sequential stages of fertilization in Fucus. These observations are discussed in terms of analogies with fertilization in the sea urchin.