Bacterial genes responsible for the synthesis of the aliphatic polyester compound, poly-D-(-)-3-hydroxybutrate (PHB) were introduced into cotton (Gossypium hirsutum L. Cv DP50) through particle bombardment. The resulting transgenic cotton contained PHB in the cotton fiber lumen. The presence of PHB granules in transgenic fibers resulted in measurable changes in thermal properties. The fibers exhibited better insulating characteristics than natural cotton. The rate of heat uptake was higher and cooling slower in transgenic fibers. Thus, the transgenic fibers had higher heat capacity and lower thermal conductivity. These results demonstrate that thermal properties of fibers can be enhanced by genetic engineering of cotton. Based on our preliminary studies of mixtures of PHB and cellulose powder, it appears that the biological composite of cellulose, PHB and, probably, bound water are necessary for the enhanced heat uptake of transgenic fibers. The heat uptake may be influenced by interactions of PHB within the fiber lumen.