It is well known that the atmosphere is a conveyor of microorganisms, and that bacteria can act as ice or cloud condensation nuclei, but clouds have not been considered as a site where organisms can live and reproduce. Here we show that bacteria in cloud droplets collected at high altitudes are actively growing and reproducing at temperatures at or below 0 ¿C. Since ~60% of the earth surface is covered by clouds, cloud water should be considered as a microbial habitat. ¿ 2001 American Geophysical Union