The fertilization of the fern Pteridium aquilinum var.latiusculum are observed using electronic microscopy.(1)The spermatozoid penetrates the egg through the fertilization pore.The gyratory motion of the spermatozoid serves to form a fertilization cavity just beneath the fertilization pore and then enters the egg cytoplasm.(2)The prolonged gyratory motion of the first spermatozoid and irritability reaction caused by the sperm penetration may contribute to block the polyspermy.(3)The nuclear evaginations persist 12 h and then detached from the nuclear body and finally are digested in the zygote cytoplasm.(4)The zygote establishes a horizontal polarity by vacuolization of the cytoplasm in the posterior part of the zygote before the zygote divides.The present study provides some new insights into the cytological mechanism of fertilization in ferns,which contribute to understand the cytological mechanism of the fertilization and evolution of the sexual reproduction of the ferns.