Liberibacters form a Rhizobiaceae clade of phloem-limited pathogens of limited host range. Two obligately parasitic species have been sequenced: Candidatus Liberibacter asiaticus (Las), which causes citrus Huanglongbing (HLB) world-wide, and Ca. L. solanacearum (Lso), which causes potato "zebra chip" disease. A third species, Liberibacter crescens (Lcr), was isolated from mountain papaya, grown in axenic culture and sequenced. In an effort to identify common host determinants, the complete genomic DNA sequence of a second HLB species, Ca. L. americanus (Lam) strain "São Paulo" was determined. The circular genome of 1,195,201 bp had an average 31.12% GC content and 983 predicted protein encoding genes, 800 (81.4%) of which had a predicted function. There were 658 genes common to all sequenced liberibacters and only 8 genes common to Lam and Las but not found in Lso. Surprisingly, most of the lipopolysaccharide biosynthetic genes were missing from the Lam genome, as well OmpA and a key regulator of flagellin, all indicating a Lam strategy of avoiding production of major pathogen-associated molecular patterns (PAMPs) present in Las and Lso. As with Las, one of two Lam prophages replicated as an excision plasmid and carried potential lysogenic conversion genes that appeared fragmentary or degenerated in Lso.