Colloidal silver iodide prepared in several different ways has been examined by electron microscopy. It was found that the particles exhibited well-defined shapes and varied in size from 10-5,000 A according to the method of preparation. Electron microscope observations were supported by the examination of single particles using the microdiffraction technique; this revealed that such particles were normally single crystals. Under the electron optical conditions of the Siemens Elmiskop, two types of phenomenon were observed, namely, highly mobile changes of contrast within the particle and the formation of filaments. The filaments also showed changes of contrast during growth and after formation often exhibited a banded structure. The preparation of thin flat particles of silver iodide enabled some preliminary electron microscope observation of internal structure to be carried out directly. Possible mechanisms for the phenonema observed are discussed.