The dependence of the superconducting transition temperature inGdBa2Cu3O6+&dgr;single crystals on the magnetic fieldHapplied parallel to the intercrystalline layers is studied experimentally. A nontrivial dependence of the superconducting transition temperature on the fieldHis explained by magnetic field penetration into high-Tcsingle crystals. The presence of a layered structure leads to the formation of a spatially periodic magnetic structure with a period varying discretely in fieldsH≫Hc1(Hc1is the lower critical field). Any change in the magnetic structure results in a nonmonotonic (jumpwise) dependence of the superconducting transition temperature on the fieldH.The effect is estimated numerically by using the results of calculation of the critical temperature of small samples in strong magnetic fields. ©1998 American Institute of Physics.