  We have measured the form factor ratios r_V = V(0)/A_1(0) and r_2 =
A_2(0)/A_1(0) for the decay D_s^+ -> phi ell^+ nu_ell, phi -> K^+ K^-, using
data from charm hadroproduction experiment E791 at Fermilab. Results are based
on 144 signal and 22 background events in the electron channel and 127 signal
and 34 background events in the muon channel. We combine the measurements from
both lepton channels to obtain r_V = 2.27 +- 0.35 +- 0.22 and r_2 = 1.57 +-
0.25 +- 0.19.
