Using the CLEO detector at the Cornell Electron Storage Ring (CESR), we have determined the ratio Γ(${\mathit{D}}_{\mathit{s}}^{+}$→φ${\mathit{l}}^{+}$ν)/Γ(${\mathit{D}}_{\mathit{s}}^{+}$ →φ${\mathrm{\pi }}^{+}$)=0.49±0.${10}_{\mathrm{-}0.14}^{+0.10\mathrm{}}$. We use this measurement to derive B(${\mathit{D}}_{\mathit{s}}^{+}$→φ${\mathrm{\pi }}^{+}$).

• Received 28 June 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.1531