Isotope branching in the F + HD reaction is investigated at ultracold temperatures. It is found that in the limit of zero temperatures, the rate coefficient for HF formation in the F + HD (v = 0, j = 0) reaction is a factor of 5.5 greater than that of DF formation. The difference is attributed to the more efficient tunneling of the lighter atom. For HD molecules initially in the j = I excited rotational level, a clear preference is seen for chemical reaction leading to HF and DF products with an HF/DF branching ratio of 5.2 rather than for nonreactive rotational quenching.