Segmentation is a prerequisite for 3-D visualization of image volumes. It has turned out to be extremely difficult to formalize for automatic computation. We describe an interactive segmentation method that circumvents this difficulty by using low level segmentation tools, which are interactively controlled by a human user via 3-D display. Segmentation tools implemented so far are simple thresholding and morphological operations. The method has been implemented on a workstation under UNIX using an X-Window interface based on the OSF/MOTIF toolkit. It is shown with examples from different applications that this simple approach delivers good results in only a short amount of time.