The alpha-amylase excreted by the antarctic bacterium Alteromonas haloplanctis was purified and the corresponding amy gene was cloned and sequenced. N- and C-terminal amino acid sequencing were used to establish the primary structure of the mature A. haloplanctis alpha-amylase which is composed of 453 amino acids with a predicted Mr of 49,340 and a pI of 5.5. Three Ca2+ ions are bound per molecule and its activity is modulated by chloride ions. Within the four consensus sequences, Asp-174, Glu-200, and Asp-264 are the proposed catalytic residues. The psychrotrophic A. haloplanctis alpha-amylase is characterized by a high amylolytic activity at low temperatures, a reduced apparent optimal temperature, and typical thermodynamic activation parameters A. haloplanctis alpha-amylase has also a low thermal stability as demonstrated by the temperature effect on both activity and secondary structure. It is suggested that structure flexibility and lower sensitivity of secondary structure to temperature variations in the low temperature range are the main structural adaptations of the psychrotrophic enzyme. The unusual stacking of small amino acids around the catalytic residues is proposed as a factor inducing active site flexibility and concomitant high activity of the enzyme at low temperatures.