The paper describes observations with a 40-240 McJs radio spectroscope leading to the discovery and preliminary investigation of harmonics in the spectra of sporadic bursts from the SWl. It is found that spectral features of bursts are commonly duplicated with a 2: 1 frequency separation. The ratio is sometimes appreciably lower though apparently never greater. This and other features of the spectra are shown to support the hypothesis that the fundamental frequency corresponds to the natural plasma frequency of the corona in the vicinity of the source. By applying this result to a standard model of the corona, information on the position, velocity, and size of the sources is deduced. The results suggest that the generation of bursts may be associated with longitudinal plasma oscillations excited by fast streams of charged particles.