The principal refractive indices of brushite (CaHPO4 center dot 2H(2)O) crystals for sodium light, 589nm, have been determined by a combination of several methods: microscopic interferometry, retardation measurements with an Ehringhaus 6 lambda compensator, and determination of the angle 2V between optic axes from the measurements of extinction directions using a spindle stage and evaluating the data with the computer program EXCALIBRW. The following values were found: n(alpha) = 1.54089, n(beta) = 1.54620, n(gamma) = 1.55191, and 2V= 88.2 degrees. The standard deviations of the three indices lie in the range 7-8 x 10(-5). The results are regarded valuable in connection with studies of crystal growth kinetics in biological mineralization and related fields. (C) 2007 Elsevier B.V. All rights reserved.