A cytogenetical study of 25 Lepilemur from the north of Madagascar has given evidence for six karyotypes (with four diploid numbers). Four of the karyotypes with these diploid numbers — 2N = 38, 2N = 36, 2N = 34 — we believe evolved from one another by centric fusion; and two karyotypes resulted from the hybridization of two of these (2N = 37 and 2N = 35). The different karyotypes distinguish themselves clearly from those of other species of Lepilemur. We have named a new species, Lepilemur septentrionalis (sp. nov.) with four subspecies: L. s. andrafiamenensis (2N = 38), L. s. ankaranensis (2N = 36), L. s. sahafarensis (2N = 36), and L. s. septentrionalis (2N = 34).