Bacteria which produce freezing nuclei active at temperatures warmer than −10°C have been isolated from leaves, water and snow. Those which produce freezing nuclei active at the warmest temperatures include strains of P. fluorescens and P. syringae. Only cultures of P. fluorescens were shown to be active in a cloud simulator. Activities as high as one freezing nucleus per bacterial cell were observed, while activities of one nucleus per 20–100 cells were more common. The nucleating activity appears to be associated with the cell wall or cell wall fragments of the bacteria.