One method of testing the effect of a fungicide on the growth of fungal mycelium in vitro involves implanting small discs of mycelium into agar plates impregnated with fungicide. Growth is measured as the increase in colony diameter with time, but it is possible for some growth to occur without a corresponding increase in colony diameter. Two empirical models, involving an upper threshhold dose, are considered for this situation. A 3-parameter model based on the exponential curve is found to provide a satisfactory fit to most of the sets of data considered.