We describe identification and characterization of a novel two-copy gene of the parasitic protozoan Leishmania that encodes a nuclear protein designated LNP18. This protein is highly conserved in the genus Leishmania, and it is developmentally regulated. It is an alanine- and lysine-rich protein with potential bipartite nuclear targeting sequence sites. LNP18 shows sequence similarity to H1 histones of trypanosomatids and of higher eukaryotes and in particular with histone H1 of Leishmania major. The nuclear localization of LNP18 was determined by indirect immunofluorescence and Western blot analysis of isolated nuclei by using antibodies raised against the recombinant protein as probes. The antibodies recognized predominantly a 18-kDa band or a 18-kDa-16-kDa doublet. Photochemical cross-linking of intact parasites followed by Western blot analysis provided evidence that LNP18 is indeed a DNA-binding protein. Generation of transfectants overexpressing LNP18 allowed us to determine the role of this protein in Leishmania infection of macrophages in vitro. These studies revealed that transfectants overexpressing LNP18 are significantly less infective than transfectants with the vector alone and suggested that the level of LNP18 expression modulates Leishmania infectivity, as assessed in vitro.