The thermopower of highly conducting quarter-filled-band TCNQ (tetracyanoquinodimethane) salts corresponds to an entropy per carrier of k ln2. We have measured the change of the thermoelectric power upon application of a magnetic field for quinolinium ditetracyanoquinodimethane. We find an isotropic shift which agrees with the entropy change of a free-spin system. Coupled with the observation of a negligibly small magnetoresistance, this provides definite evidence that the conducting electrons are correlated as in the strong-coupling Hubbard model. © 1979 The American Physical Society.