Uniaxial and equibiaxial tensile strengths are reported for glass‐ceramic specimens exhibiting strength isotropy. Uniaxial strengths were determined in both 3‐ and 4‐point bend tests. Stressing rate was a controlled variable and the tests were conducted in both dry N2and water environments, to provide data for different conditions of slow crack growth. There was little or no difference between strengths in the 3‐ and 4‐point bend tests, indicating the absence of a size effect. In comparable tests, the strength under equibiaxial tension was greater than under uniaxial tension. The biaxial strengthening became less pronounced with increased slow crack growth during testing. A maximum observed biaxial‐to‐uniaxial strength ratio of 1.21 resulted from ball‐on‐ring and uniaxial bend tests in dry N2. These results are attributed to differences in flaw severity between biaxial and uniaxial stressing. The decreasing strength ratios can be explained by a change in configuration of critical flaws due to slow crack growth. Numerical calculations, using a simplified two‐dimensional model of the strength‐controlling flaw, supported this hypothesis. The calculations also indicate that the slow crack growth exponent is smaller for the biaxial tension mode of stressing than