Methods previously used to study hydrophobic interaction in ordinary water have been used to measure the free energy of hydrophobic interaction between hydrocarbon chains in deuterium oxide at 25°:     (1) Conductance measurements on micellar solutions of a series of hexadecyltrimethylammonium carboxylates give the effect of hydrocarbon chain length on the free energy of binding a counter-ion (carboxylate ion) to the micelle, and hence the free energy of hydrophobic interaction.     (2) Comparison of the kinetics of the reaction between decylamine and 4-nitrophenyl decanoate with the kinetics of the corresponding reaction without hydrophobic side chains (the reaction between ethylamine and 4-nitrophenyl acetate) gives the extent to which hydrophobic interaction stabilizes the transition state. This then provides another estimate of the free energy of hydrophobic interaction.     The free energy of hydrophobic interaction in D2O was found to be - 1.76 kJ mol-1, for each contact between two methylene groups, compared with -1.40 kJ mol-1 in H2O. Hydrophobic interaction is therefore stronger in D2O than in H2O.