When grown under a variety of axenic conditions gametophytes ofE. arvense, E. telmateia, E. sylvaticum, E. palustre, andE. fluviatileare all morphologically distinct. Lamellae from archegonial branches provide several diagnostic features but lamellae associated with antheridia are much less distinct. The archegonia of all five species have two neck canal cells and three tiers of neck cells and are of limited use taxonomically. Although antheridium development is basically similar in all species, during the metamorphosis of the spermatocytes specific differences arise in the shape of the cavity, degree of emergence and the number of opercular cells. A taxonomy of the gametophytes based on antheridium morphology corresponds very closely to the classification of the subgenusEquisetuminto three sections based on the coning behaviour of the sporophytes.