Formal specification has long been advocated but rarely practised. When practised, it is often applied to simple, already well understood objects such as stacks and other basic data types. This note begins to explore the issue of formal specification within an object-oriented environment. We attempt to specify formally the object Bitmap. This is a particularly interesting choice, for bitmaps are mutable (i.e., they change in time), they can have a perceived effect on images, and their semantics is highly dependent on context. Bitmaps are therefore in many ways worst-case problems for formal specification.