An anechoic lining for underwater sound use can be made from waterproofed, rubberized, horsehair batting. It decreases the reflectivity 20 db below that obtained from a steel wall over a frequency range from 100 kc to 400 kc and for a wide range of angles of incidence. It is inexpensive, easily installed, and can be used for several 8‐hour periods before additional waterproofing is necessary.