In this communication a theoretical relation is derived for the travel time of the proton whistlers to reach the satellite from their place of occurrence. The expression for the group travel time contains two terms. The first term is the dominant term and is inversely proportional to (&OHgr;-&ohgr;)1/2, where &OHgr; and &ohgr; are the proton gyrofrequency and the wave frequency, respectively. The contribution from the second term, which is inversely propoprtional to (&OHgr;-&ohgr;)7/2, is about 10% of the first term. It is shown that the group travel time is sensitive to the variation of temperature, gradient of proton gyrofrequency, and ion density along the path of a proton whistler. The effect of variation in each of the parameters on the group travel time is discussed, and it is shown that an estimate of temperature can be made from the measured data on proton whistlers.