The behavior of clathrate hydrate formation was examined at the interface between hydrochlorofluorocarbon liquid, HCFC-141b, and a shear water flow parallel to the interface. Large polygonal and transparent hydrate-crystal plates were found to grow anisotropically at a low degree of subcooling, defined as the difference between the three-phase [water-HCFC-141b (liquid)-hydrate] equilibrium temperature at atmospheric pressure and the temperature of the water. Hydrate-crystal aggregates were found to grow isotropically at a high degree of subcooling. Tiny hydrate-crystal aggregates were formed by mutual collisions of the aggregates and/or by collapses of the hydrate-crystal aggregates when the water flow rate was high. The hydrate-crystal aggregates tended to coalesce when the degree of subcooling was high and the flow rate was low. The growth rate of the hydrate-crystal aggregates depended strongly on the degree of subcooling and weakly on the rate of heat transfer between the water and the aggregate surface parallel to the interface. (C) 2002 Elsevier Science B.V. All rights reserved.