Third virial coefficients (A(3)) are reported for a series of high molecular weight, narrow molecular weight distribution polystyrenes (PS) in benzene, a good solvent, and three Theta solvents : cyclohexane (CH), 1-chloroundecane (CUD), and diethyl malonate (DEM). A(3) values measured in benzene and cyclohexane are in good agreement with previously reported results for these polymer/solvent systems. A(3) values measured in CUD and DEM are consistently much smaller than values measured in CH for the same sample. This specific solvent effect is also reflected in larger Theta condition intrinsic viscosity ([eta]) values for PS in CH, as compared to PS in CUD or DEM. These results demonstrate a correlation between larger repulsive three-body interactions under Theta conditions and larger Theta condition [eta] values. However, the radii of gyration of PS samples in these three Theta solvents were identical within experimental error. These results suggest that hydrodynamic properties are more sensitive to residual ternary interactions than are thermodynamic properties. Furthermore, our results indicate that the Flory hydrodynamic parameter phi can have different values in different Theta solvents. Similar results were reported recently by Yamakawa and co-workers for poly(methyl methacrylate) in two different Theta solvents.