The strength of zirconium diboride (ZrB2) ceramics was measured up to 2300 degrees C, which are the first reported measurements above 1500 degrees C since 1970. ZrB2 ceramics were prepared from commercially available powder by hot pressing. A mechanical testing apparatus capable of testing material in the ultra-high temperature regime with atmosphere control was built, evaluated, and used. Four-point bend strength was measured as a function of temperature up to 1600 degrees C in air and between 1500 degrees C and 2300 degrees C in argon. Strength between room temperature and 1200 degrees C was 390 MPa, decreasing to a minimum of 170 MPa between 1400 degrees C and 1500 degrees C, with strength increasing to 220 MPa between 1600 degrees C and 2300 degrees C.