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An original adaptive control method is presented for controlling a nonlinear multivariable system. The method, a modified quasi-linear approach, involves dividing the source excitation into a series of pulsing rounds and is implemented as a control algorithm on a computer. The theory underlying the method is developed with reference to an application involving temperature control in interstitial laser hyperthermia. The method is both successful and necessary to achieve optimally uniform elevated temperatures in a ground beef phantom. Apart from variable and parameter definitions, the method is otherwise general and might be useful for controlling a nonlinear system in which no prior exact characterization of the system is possible. Simulations were conducted to assess the effectiveness of the method in systems for which the unit excitation response changes by factors ranging from zero to three over the total period of excitation. In each case the method has proven stable.