We have observed Josephson coupling between CuO double layers in Bi2Sr2CaCu20& single crystals by direct measurements of ac and dc Josephson effects with current flow along the r axis. The results show that a small Bi2Sr2CaCu20& single crystal behaves like a series array of Josephson junctions which can exhibit mutual phase locking.PACS numbers: 74.50.+r, 74.60.Jg, 74.70.jm The theory of layered oxide superconductors focuses on the possible types of coupling within the essential structure elements: the CuO planes. Ho~ever, independent of the exact nature of the pairing mechanism within the layers, the t'nterlayer coupling determines most of the superconducting properties of a real crystal. To date it is not clear how the Cooper pairs move across the interlayer spacing. In Bi2Sr2CaCu20s (BSCCO) the upper critical magnetic fields are so large that the standard anisotropic Ginzburg-Landau theory yields c-axis coherence lengths g, of the order of I A [I]. This is much less than the distance of 12 A between the CuO double layers. Therefore, the Lawrence-Doniach picture would be more appropriate [2], i.e., BSCCO should be described as a set of discrete superconducting layers whose order parameters are coupled by Josephson interaction. Several theories [2,3] and recent experiments concerning the angle dependence of the critical current [4] and the torque [5] support this idea. A convincing proof, however, can only be given by direct measurements of ac and dc Josephson efl'ects [5]. The simplest model of a BSCCO superconductor is a stack of superconducting sheets consisting of the CuO bilayers separated by the BiO and SrO layers acting as weak links or as insulators. Experimentally we therefore have to consider series arrays of proximity [superconductor-normal-metal-superconductor (SNS)] or tunneling [semiconductor-insulator-semiconductor (SIS)] Josephson junctions and pay attention to the following points.(I) If the c-axis supercurrents are limited by the Josephson effect, the I-V characteristics exhibit a concave curvature in the resistive state. If, on the other hand, the supercurrents are limited by flux flow, this curvature is convex. Therefore the I-V characteristics give first-hand information whether or not the supercurrents are of the Josephson type.(2) If the (ab)-plane dimensions of the Josephson junctions are larger than the Josephson penetration length A, j, Josephson vortices can be formed in the barrier. Whereas in standard tunnel junctions k~i s governed by the magnetic self-fields of the Josephson currents, in BSCCO kj is dominated by the kinetic momentum of the current flow along the (ab) axes, because the thickness of the electrodes (i.e., the CuO double layers) is much smaller than the field penetration depth along the e axis (A. ,q = 3000 A) [6]. The Lawrence-Doniach theory yields A, j=yt, where t is the interlayer spacing and y is the penetration depth anisotropy ratio [7]. For BSCCO k, = 100 pm [8], which yields X~= 0.4 pm.(3) In standard tunnel junctions with an electrode thickness larger than ...