AbstractBacteria exhibit unique diversity in their ability to grow at different temperatures. Indeed, eubacteria and archaebacteria are the only organisms able to grow above 65d`. The temperature range for a species is generally considered to be a stable character; however, mutants may be isolated mat have a Tminor Tminbelow or above the parent organism. Some bacteria may also be coaxed to grow at different temperatures by training cultures, through an incremental increase or decrease of temperature. Genetic approaches, for example, the transformation of mesophilicBacillusto thermophily using DNA from closely related thermophiles, has been very controversial. A major problem has been the lack of stability of the high-temperature phenotype upon subculture, which has not allowed extensive genetic and biochemical characterization of the transformants. The mechanism whereby the thermophilic phenotype is carried is unknown, although it is possible that the adapter genes are plasmid encoded. Studies using phenotypically stable transformants indicated that the thermostability of some cellular components was significantly increased, both in the vegetative cell and spore state. Enzyme thermostability, for example, appeared to be associated with an increased use of hydrophobic amino acids; however, the biochemical mechanisms for these alterations remain unknown. Thermophily is still a challenging problem with some interesting molecular biology.