The recent discovery of superconductivity at 11.5 K in the graphite intercalation compound (GIC) CaC6 has opened new perspectives in the physics of graphite. One of the main open questions for superconducting GICs is related to the nature of the pairing mechanism, since the possibility of an unconventional, excitonic or plasmonic, origin of superconductivity has also been invoked as an alternative to a simple electron-phonon interaction. To better understand the origin of pairing mechanism in these compounds, a first step is to determine the symmetry of the superconducting gap function and the nature of the elementary excitations. To this aim, we have performed the first high-resolution measurement of the in-plane magnetic penetration depth lambda(ab)(T), in a c-axis oriented polycrystalline CaC6 bulk sample using a high-resolution mutual inductance technique. A clear exponential behavior Of)ab(7) has been observed at low temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the standard BCS theory yields lambda(ab)(0) = (720 +/- 80) angstrom and Delta(0) = (1.79 +/- 0.08) meV. The ratio 2 Delta(0)/k(B)T(C) = (3.6 +/- 0.2) gives therefore indication for a conventional weakly coupled superconductor. By using these results as fixed parameters, a BCS calculation on the superfluid density in the overall temperature range shows that the sample under test lies in the local dirty limit. (C) 2007 Elsevier B.V. All rights reserved.