The observations that terrestrial ice nuclei can have a biogenic origin and that certain bacteria can initiate freezing at exceptionally warm temperatures (−1.5°C) are now well documented. Less well understood are active ice nuclei (−2 to −5°C) found in sea water, marine fog and the marine atmosphere in general. Recently, the authors have isolated an ice nucleation-active (INA) bacterial strain (FB 1032) from cultures of the marine dinoflagellate, Heterocapsa niei. FB 1032 is halotolerant and phenotypically similar to Pseudomonas fluorescens biotype G, although it differs from biotype G strains in its bacteriophage sensitivity and expression of the INA phenotype. A search for the INA pseudomonad in sea water at La Jolla, California was unsuccessful, but several INA Erwinia sp. were isolated. The possible terrestrial origin of these INA bacteria is suggested.