The essential requirements of fabrics for maintaining the body at normal temperature under tropical conditions are shown to be high reflecting and emissive powers, low transmitting power for radiant energy, and high permeabillty to moisture. Some experimental methods are described by which the thermal properties of fabrics may be studied. Using one typical fabric made for export to the Tropics, it is shown that the total heat reflected is approximately equal to the total heat, transmitted, thus indicating the relative importance of these two modes of heat transference. A method is described by means of which the amounts of heat transmitted by fabrics may be compared, and the results of tests on a range of materials typical of those exported to the tropics are given to indicate the scope of the method. Testing the fabrics in the dry state, it was found that the most transparent allowed about five times as much heat to pass as the least transparent, this being due to differences in construction, involving such matters as filling, weave and so forth. There was a certain degree of irregularity from place to place along the length of a fabric in the heat transmitted, but this irregularity was independent of the amount of heat transmitted. The effect of washing the fabric has been determined in relation to the loss of weight and the amount of heat transmitted. The greatest increase in heat transmitted occurred when most filling was removed, and heavily filled fabrics lost their heat insulating value almost entirely on washing. Some fabrics which had not been filled transmitted less heat after washing, owing to shrinkage. The most suitable of the fabrics tested for tropical use appeared to be one which was heavy and affected little by washing. A method for comparing the total heat reflecting powers of fabrics has yet to be devised. The effect of the moisture content of a fabric on its transmitting and reflecting powers has been studied. The heat transmitted decreased, with the four fabrics examined, in direct proportion to the increase in relative humidity of the atmosphere with which the fabric was in equilibrium. The mean decrease on changing from dryness to saturation was 22.7 per cent., but the decrease was greatest with those fabrics which transmitted least heat when dry. The effect of moisture content on the heat reflecting power was only examined in one case, but appeared to be insignificant.