The specific growth rates at various temperatures of 12 bacterial species were measured and plotted as Arrhenius profiles. Temperature characteristics and optimum temperatures for maximum specific growth rates were estimated from these curves. The data reveal that one of two forms of the Arrhenius profile is characteristic of each bacterium: one curve is a simple smooth curve with a single predominant slope at sub-optimum temperatures; the other is a more complex curve with two distinct slopes at sub-optimum temperatures. The simple curves describes bacteria across the entire biokinetic range whereas the more complex curve occurs only with organisms which have optimum temperatures for replication above 37 degrees C. Describing bacteria in terms of these forms of the Arrhenius profile is less arbitrary than is categorization based on optimum temperatures.