As a contribution to the discussion about the nature of noctilucent clouds some aspects of the ice particle hypothesis are considered. A model of an ice cloud is computed, assuming a temperature profile and wave motions in agreement with observations. Some of the observed characteristics of noctilucent clouds are well reproduced by the model. Similar computations for stratiform clouds show that a diffusion coefficient of the order 107 cm2/sec seems to be required to keep particle sizes within the observed range. It is concluded that the ice particle theory and the cosmic dust theory should be considered as equally important in future studies of noctilucent clouds. DOI: 10.1111/j.2153-3490.1962.tb01339.x