Underwater ultrasonography is a useful tool for studying structure and mechanical events in live, fairly large marine animals that do not possess an unduly thick, calcified integument. The limit of discrimination with present technology is about 2 mm, so fine detail is absent; the transducer has to be within 1–2 cm of the animal being studied or attenuation of the ultrasound by sea-water will restrict penetration of tissues. The non-invasive nature of the technique makes it ideal for the study of valuable or particularly delicate animals (e.g. Eledone); there would also appear to be considerable potential for use of ultrasonography as a teaching/demonstration aid in marine biology.