Three new species ofDasylirion and a new variety ofD. longissimum Lem. from the mountains of northern Mexico are described and illustrated. Two of the new species are from northwestern Mexico:D. gentryi, with large, rosecolored fruits and glossy, non-waxy leaves; andD. sereke, characterized by roundish fruits with an unusuall large style.Dasylirion miquihuanense is a new species from northeastern Mexico with narrow, strictly upright leaves and widely separated marginal prickles. TheDasylirion longissimum complex consists of two species,D. quadrangulatum S. Wats andD. longissimum Lem., characterized by narrow, quadrangulate leaves with reduced or absents marginal prickles. Distinguishing features of these two species, which have often been confused, are presented andD. longissimum var.treleasei, characterized by small flowers and fruits, is described.