The study of leaf nodulation in several tropical plant families dates back to the beginning of the 20th century. Various genera of bacteria, e.g., Phyllobacterium, were thought to be associated with leaf nodules (see below). Phyllobacterium was described by Knösel (1962) but the genus only found wide taxonomic recognition with the publication of Bergey’s Manual of Systematic Bacteriology (Knösel, 1984). DNA-rRNA hybridizations revealed a close relationship between Phyllobacterium and bacteria from the Centers for Disease Control (CDC) group Vd within the “alpha” subclass (rRNA superfamily IV) of the proteobacteria together with those throughout of the genera Brucella, Rhizobium, Mycoplana, and Agrobacterium (De Ley et al., 1987; Stackebrandt et al., 1988). Holmes et al. 1988 proposed the name Ochrobactrum anthropi for CDC group Vd. They also demonstrated that Phyllobacterium showed closest phenotypic similarity to Ochrobactrum anthropi. In the present chapter, the genera Phyllobacterium and Ochrobactrum are treated together because of their close phylogenetic relatedness. Nevertheless, the ecological niches from which these bacteria have been isolated are quite different: Phyllobacterium strains occur in leaf nodules and in the rhizosphere, whereas Ochrobactrum strains have been isolated almost exclusively from human clinical material. In clinical microbiology, Ochrobactrum, more recently described as Achromobacter group Vd until formally named, was usually treated together with Achromobacter xylosoxidans (more recently known as Alcaligenes xylosoxidans subsp. xylosoxidans) (Gilardi, 1978; Rubin et al., 1985). It has now become apparent that Ochrobactrum and Phyllobacterium are not related phylogenetically to Achromobacter xylosoxidans, which belongs to the Alcaligenaceae in the “beta” subclass (rRNA superfamily III) of the proteobacteria (De Ley et al., 1986).