We have measured the quasi-static yield strength of NaCl to 41 GPa at room temperature. Between 0 and 28 GPa, the strength of the B1 phase increases eightfold, from 0.08 to 0.65 GPa. Across the B1-B2 transition, the strength of NaCl decreases about 50%. We show that this result is consistent with the measurements of Bridgman <1937> on the shear strength of the B1 and B2 potassium halides to 5 GPa. We propose that the simultaneous increases in coordination and nearest-neighbor distance have opposite effects on the change in shear strength across B1-B2 transitions. Increasing the atomic coordination makes the B2 structure stronger than the B1 while increasing the nearest-neighbor distance decreases the strength of the high pressure phase. In NaCl the large increase in nearest-neighbor distance more than offsets the effect of increased coordination, and thus the B2 phase is weaker than the B1 phase. ¿ American Geophysical Union 1988