We describe an improved microvolumeter {(MVM)} for rapidly measuring volumes of small biological samples, including live zooplankton, embryos, and small animals and organs. Portability and low cost make this instrument suitable for widespread use, including at remote field sites. Beginning with Archimedes' principle, which states that immersing an arbitrarily shaped sample in a fluid-filled container displaces an equivalent volume, we identified procedures that maximize measurement accuracy and repeatability across a broad range of absolute volumes. Crucial steps include matching the overall configuration to the size of the sample, using reflected light to monitor fluid levels precisely, and accounting for evaporation during measurements. The resulting precision is at least 100 times higher than in previous displacement-based methods. Volumes are obtained much faster than by traditional histological or confocal methods and without shrinkage artifacts due to fixation or dehydration. Calibrations using volume standards confirmed accurate measurements of volumes as small as 0.06 {microL.} We validated the feasibility of evaluating soft-tissue samples by comparing volumes of freshly dissected ant brains measured with the {MVM} and by confocal reconstruction. Address correspondence to John K. Douglass, Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Ancon, Panamá, República de Panamá; address United States-originating mail to John K. Douglass, Smithsonian Tropical Research Institute, {MRC} 0580-12, Unit 9100 Box 0948, {DPO} {AA} 34002- 9998. e-mail: .
