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We present the first metallicity distribution functions of the old fieldpopulations in the Magellanic Clouds. Our metallicities are based on theFourier decomposition of Type ab RR Lyrae light curves from the OpticalGravitational Lensing Experiment (OGLE-III). On the metallicity scale of Zinn &West; we find a mean metallicity of [Fe/H] = -1.50 +/- 0.24 dex based on 16776RR Lyrae stars in the Large Magellanic Cloud (LMC). For the Small MagellanicCloud (SMC) we obtain -1.70 +/- 0.27 dex based on 1831 RR Lyrae stars. Theseuncertainties represent the intrinsic spread in the population rather than thestandard deviation of the mean. Our results are in good agreement with the fewexisting spectroscopic metallicity determinations for LMC RR Lyrae stars fromthe literature. For both the LMC and the SMC the metallicity spread exceeds 1dex in [Fe/H]. The distribution of metallicities in both Clouds is veryuniform, and no significant metallicity gradient is detectable. We also do notfind any pronounced populations of extremely metal-poor RR Lyrae candidateswith metallicities well below -2 dex, although we need to caution that thephotometric method used may overestimate the metallicities of metal-deficientstars. Moreover, because of stellar evolutionary effects one does not expect toobserve many RR Lyrae stars among very metal-poor horizontal branch stars. Wesuggest that the Magellanic Clouds experienced fairly rapid and efficient earlyenrichment involving pre-enriched gas as well as possibly gas infall, whilemetal loss through outflows does not seem to have played a significant role.Moreover we suggest that the differences in the metallicities of the oldpopulation of LMC and SMC make an origin from a single, common progenitorunlikely, unless the separation happened very early on.

Metallicity distribution functions of the old populations of the Magellanic Clouds from RR Lyrae stars