This paper presents an optical diffraction tomography technique based on digital holographic microscopy. Quantitative 2-dimensional phase images are acquired for regularly-spaced angular positions of the specimen covering a total angle of pi, allowing to built 3-dimensional quantitative refractive index distributions by an inverse Radon transform. A 20x magnification allows a resolution better than 3 microm in all three dimensions, with accuracy better than 0.01 for the refractive index measurements. This technique is for the first time to our knowledge applied to living specimen (testate amoeba, Protista). Morphometric measurements are extracted from the tomographic reconstructions, showing that the commonly used method for testate amoeba biovolume evaluation leads to systematic under evaluations by about 50%.