The morphology of the cells of an organism formerly referred to as a member of Leptospira sp. serovar illini has been studied by electron microscopy. Cells of these strains differ morphologically from the cells of leptospires in the following: (i) cells of the former have cytoplasmic tubules which leptospires do not possess, and (ii) the structure of the basal complex on the flagella of the former is similar to that of gram-positive bacteria, whereas the corresponding structure on the flagella of leptospires is similar to that of gram-negative bacteria. Pillot (J. Pillot, thesis, University of Paris, Paris, France, 1965) proposed in 1965 that leptospires should constitute a family, Leptospiraceae. However, Pillot proposed this name in a thesis, which is not considered to be effective publication. Hence, Leptospiraceae is here proposed as the name of a new family. In consequence of the new morphological evidence presented here together with the data from other publications on the deoxyribonucleic acid base composition and serological and cultural properties of cells of strains of Leptospira sp. serovar illini, it is proposed that this family should consist of two genera: Leptospira Noguchi 1917 and Leptonema gen. nov. The type species of Leptonema is Leptonema illini, and the type strain of this species is strain 3055 (= NCTC 11301).