Some bacterial species, which commonly infect humans, have developed environmental mimicry strategies by mimicking the carbohydrate structures present on human cell surfaces. A number of the enzymes involved in the synthesis of these bacterial mimics can undergo random variation. This ability to modify surface carbohydrate expression in a random manner enhances selection for the most “fit” expression for any environment that the bacteria would encounter. This chapter discusses this phenomenon in two human pathogens, Neisseria gonorrhoeae and Haemophilus influenzae. Molecular mimic glycolipid structures are important in the pathogenesis of Neisseria and Haemophilus spp. for both immune evasion and in the adherence and invasion of human cells. In several instances, this mimicry necessitates that these organisms parasitize specific sugars from the human in order to assemble the appropriate structure. Both species have developed mechanisms randomly to switch “on and off” expression of the enzymes responsible for assembly of the terminal sugars on these glycolipids by a process termed “phase variation.” This allows members of the respective population to survive in a wide range of host environments assuring continuation of transmission, colonization, and infection.