Growth of the fungusEchinodontium linctorium, cause of an important trunk rot in true firs, was studied at various fixed temperatures in vitro. A polynomial approximation of the three-dimensional growth surface over temperature and time was developed with the aid of statistical techniques and computer facilities. The growth of isolates from northern and central Sierra Nevada forest regions of California was compared in this manner. The optimum temperature of the northern isolates was found to vary with the period of observation. For these isolates, the optimum temperature for rate of growth decreased with time, approaching a constant value. The optimum temperature of the central Sierra Nevada isolates was not found to vary with time. At near-optimum temperatures, the estimated growth rate of isolates from northern California was 75% greater at 20 days than that of the central Sierra Nevada isolates. The estimated optimum temperatures of the isolates from the two regions differed by only 1.2 °C at 20 days. Deviations of 1.2 °C from the optimum produced only slight differences in the estimated growth rate of either group of isolates. Use of the constant "terminal optimum temperature" is suggested for describing the growth response of fungi. Unlike the commonly reported "optimum temperature", the value of the terminal optimum temperature does not depend upon the period of observation.