This paper describes morphological events associated with fertilization in Halocynthia roretzi, especially the acrosome reaction, apical process formation, and gamete fusion. Sperm can pass through the chorion with an intact acrosome, leaving a distinct hole in the chorion. This suggests that the sperm has nondiffusible lysins located on the sperm head: these lysins probably correspond to the fuzzy material which decorates the outer surface of the plasma membrane enclosing the sperm head. A new type of acrosome reaction occurs in the perivitelline space. Apical processes protrude only from the sperm apex of acrosome-reacted sperm. This suggests that the acrosome reaction is a prerequisite step for apical process formation. Gamete fusion occurs between the sperm apical processes and the egg plasma membrane. Stick-like structures which are uniform in shape and size ornament the outer surface of the follicle cells. This is the first report to describe surface ornamentation of the follicle cells and its possible role during fertilization. (C) 1995 Academic Press, Inc.