The Unified Radio and Plasma wave Experiment (URAP) on the Ulysses spacecraft provides in situ observations of Langmuir waves and ion-acoustic waves in the solar wind. The observations presented in this paper were obtained at 5.2 AU from the Sun. Low-frequency (20--200 Hz in the spacecraft frame) electric field signals are observed coincident in time with the most intense Langmuir waves. The low-frequency wave signals are identified as long-wavelength ion-acoustic waves. These observations provide evidence for the decay of Langmuir waves into daughter Langmuir and ion-acoustic waves (the electrostatic decay process) in the solar wind.