The containment and stability of the theta pinch depend critically upon the value of beta. However, while theoretical calculations predict a value of unity on the axis, experimental results show that the value on axis is substantially less than this.The possibility that these low experimental values are due to a small field diffusion in the early stages of the current sheath formation, followed, during the compression, by an energy loss, is examined. Using various values of γ to simulate the effects of energy loss, the variation of beta during the shock phase, and during the isentropic compression phase, is calculated.The results are compared with observations on a Megajoule Theta pinch and it is shown that the penetration of a few hundred gauss during the sheath formation stage can account for the observed values of beta.