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The apical regions of motile Limulus spermatozoa readily adhere to the outer layer of the egg envelope. Shortly after this adherence or primary attachment, the sperm acrosome reaction occurs, resulting in a stronger adhesion (secondary attachment). A sperm attachment assay that quantified the number of spermatozoa attaching to egg sections was utilized to identify components involved in primary attachment. The number of spermatozoa attached was examined after treatment of either egg sections or spermatozoa with various compounds. Egg sections treated with asparagus pea lectin (250 micrograms/ml) bound significantly fewer spermatozoa as compared to those exposed to wheat germ agglutinin, concanavalin A, and garden pea lectin. Furthermore, sperm attachment was also greatly reduced when egg sections were first incubated with the glycosidase, alpha-L-fucosidase (less than or equal to 5% of controls). Treatment of spermatozoa with alpha-L-fucose, fucoidan, or p-aminophenyl fucoside also reduced sperm attachment when compared to Millipore-filtered artificial seawater controls. Egg sections were treated with fluorescein-conjugated lectins to confirm that the lectins actually bound to portions of the egg envelope and that various sugars are present in the egg envelope. Evidence suggests that the methylpentose, alpha-L-fucose, plays an important role in primary sperm attachment in Limulus.