We report the discovery of a significant population of RR Lyrae variables in the dwarf spheroidal galaxy Leo I. Based on 40 V and 22 B images of the galaxy taken using the ESO Wide Field Imager we have identified so far 74 candidate RR Lyrae's in two CCD's hosting the main body of the galaxy. Full coverage of the light variations and pulsation periods have been obtained for 54 of them, 47 of which are Bailey {\it ab}-type RR Lyrae's (RRab's) and 7 are {\it c}-type (RRc's). The period distribution of the presently confirmed sample of RRab's peaks at P=0\fd60, with a minimum period of 0\fd54. The pulsational properties indicate for Leo I an intermediate Oosterhoff type, similar to other dwarf galaxies in the Local Group and the LMC. However, the rather long minimum period of the {\it ab}-type variables, and the significant number of RRab's with long period and large amplitude, suggest that the bulk of the old population in Leo I is more like the Oosterhoff type II globular clusters. The most straightforward interpretation is that a range in metallicity is present among the RR Lyrae's of Leo I, with a significant population of very metal-poor stars. Alternatively, these OoII variables could be more evolved. The average apparent magnitude of the RR Lyrae's across the full cycle is $= 22.60 \pm 0.12$ mag, yielding a distance modulus $(m-M)_{V,0}= 22.04\pm 0.14$ mag for Leo I on the ``long'' distance scale.

Accepted for publication in ApJL; 4 pages including 3 figures