When the micropyle area of salmonid (trout and salmon) eggs was observed continuously from the moment of insemination, spermatozoa were seen moving along the surface of the chorion and entering the micropyle one by one in a directed fashion. The ability of spermatozoa to enter the micropyle was reduced after the treatment of chorions with pronase; this reduction in sperm entry was observed even before the outer opening of the micropyle channel was narrowed due to gradual swelling of the chorion by pronase treatment. Herring spermatozoa, unlike spermatozoa of most other marine fishes, were motionless in seawater. However, they became vigorously motile on contact with the micropyle area of the herring egg chorion and entered the micropyle rapidly and efficiently. Motility initiation of herring spermatozoa in the micropyle area was dependent on extracellular calcium and potassium. Sodium also appears to be intricately involved in this process as demonstrated by the initiation of sperm movement in sodium-free seawater. When herring eggs were treated with acidic seawater, organic solvents, or glutaraldehyde, spermatozoa did not initiate movement in the micropyle area, and sperm entry was not observed. Herring spermatozoa did not initiate movement in the micropyle area of salmonid eggs. These and other observations suggest that the micropyle areas of salmonid and herring eggs possess some sperm guidance factors which facilitate entry of homologous spermatozoa into the micropyle.