Microwave spectra have been observed with Fourier transform and cold jet expansion techniques in the 6-17 GHz range for trans-3,4-difluorocyclobutene, its two C-13(1) modifications, two d(1) modifications, and the perdeutero species. Watson-type Hamiltonians have been fit with a full set of quartic centrifugal distortion constants for all species except the d(4) species. Complete structures fit with an r(s)/r(0) treatment and an r(0) treatment are in satisfactory agreement. Preferred r(s)/r(0) results with Costain uncertainties in parentheses are 1.349(4) Angstrom for the C=C bond, 1.503(9) Angstrom for the contiguous C-C bonds, 1.534(4) Angstrom for the distant C-C bond, 1.398(6) Angstrom for the C-F bond, 1.081(3) Angstrom for the =C-H bond, and 1.102(5) Angstrom for the -C-H bond. Consistent with its C-2 symmetry, the ring is slightly puckered, and the C-F bonds are,roughly equatorial. Compared With the structure of cyclobutene, the structure of trans-3,4-difluorocyclobutene shows a modest lengthening of the C=C bond and more significant decreases in the C-C bond lengths. These effects are consistent with observations for other fluorine-substituted rings in which. contiguous fluorine atom substitution causes CC bond shortening and distant fluorine atom substitution causes CC bond lengthening. The CC bond adjustments in trans-3,4-difluorocyclobutene are, however, surprisingly similar to the adjustments observed for 3,3,4,4-tetrafluorocyclobutene.