Particles of cowpea chlorotic mottle virus (CCMV) underwent an abrupt structural transition as the pH was raised to near pH 6.75. At low ionic strength (I=0.2), the transition was observed as a decrease from 83S to 73S in the sedimentation coefficient of the virus particle. At high ionic strength (I=1.0), the virus disassembled to components sedimenting at about 40S. These observations suggest a structural role of the RNA in maintaining virus stability. As ionic strength at pH 7.5 was increased, a critical point was reached (I=0.4) at which the virus disassembled to the 40S aggregates. The effect of raising the temperature of both unswollen and swollen virus was to gradually, but substantially, reduce the sedimentation coefficient.