Abstract.A microbalance is used to weigh animals while they are immersed in water held at constant temperature. Paired weighings of the same individual, done in waters of identical osmolality but differing in density, permit the use of Archimedes' principle to compute animal mass and volume. Body density and weight in water can be computed from these quantities. These measurements can be used directly for developmental and morphological studies, and as points of reference in ecological or physiological studies.