We present deep Hubble Space Telescope single-star photometry of Leo A in B , V , and I .
Our new field of view is offset from the centrally located field observed by Tolstoy et al .
( 1998 ) in order to expose the halo population of this galaxy .
We report the detection of metal-poor red horizontal branch stars , which demonstrate that Leo A is not a young galaxy .
In fact , Leo A is as least as old as metal-poor Galactic Globular Clusters which exhibit red horizontal branches , and are considered to have a minimum age of about 9 Gyr .
We discuss the distance to Leo A , and perform an extensive comparison of the data with stellar isochrones .
For a distance modulus of 24.5 , the data are better than 50 % complete down to absolute magnitudes of 2 or more .
We can easily identify stars with metallicities between 0.0001 and 0.0004 , and ages between about 5 and 10 Gyr , in their post-main-sequence phases , but lack the detection of main-sequence turnoffs which would provide unambiguous proof of ancient ( > 10 Gyr ) stellar generations .
Blue horizontal branch stars are above the detection limits , but difficult to distinguish from young stars with similar colors and magnitudes .
Synthetic color-magnitude diagrams show it is possible to populate the blue horizontal branch in the halo of Leo A .
The models also suggest \approx 50 % of the total astrated mass in our pointing to be attributed to an ancient ( > 10 Gyr ) stellar population .
We conclude that Leo A started to form stars at least about 9 Gyr ago .
Leo A exhibits an extremely low oxygen abundance , of only 3 % of Solar , in its ionized interstellar medium .
The existence of old stars in this very oxygen-deficient galaxy illustrates that a low oxygen abundance does not preclude a history of early star formation .