In-situ observations have been made of ice crystals growing in solutions of trehalose and water in order to investigate how trehalose suppresses the growth of ice crystals and protects organisms from the freezing of their cellular water. In the range of trehalose concentration below 41.7 wt% the growth rate along the a-axis was found to increase proportionally to the second power of the supercooling and to be suppressed by 75% when the trehalose concentration was doubled. In this manner, trehalose proved to be approximately twice as effective as sucrose in suppressing the growth rate of ice crystals. It was also more effective than sucrose in suppressing the morphological instability of ice crystals. This effectiveness is believed to be related to the fact that trehalose binds more water molecules than does a sucrose molecule. (C) 2002 Elsevier Science B.V. All rights reserved.