From study of the Kosterlitz-Thouless-Berezinskii (KTB) transition in the superfluid density, n_s(T), of ultrathin c-axis oriented YBa_{2}Cu_{3}O_{7-delta} (YBCO) films, we find that interlayer coupling is unexpectedly strong. The KTB transition occurs at a high temperature, as if the films were isotropic rather than quasi-two-dimensional. This result agrees with a comparison of the superfluid density of YBCO with Bi_{2}Sr_{2}CaCu_{2}O_{8} and with numerical simulations of Josephson junction arrays, and challenges the thermal phase fluctuation interpretation of critical behavior near T_c in YBCO.
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