A theory for type I emission is developed based on fundamental plasma emission due to coalescence of Langmuir waves with low-frequency waves. The Langmuir waves are attributed to energetic electrons trapped in a magnetic loop over an active region. It is argued that the low-frequency waves should be generated in connection with the heating of the region. The continuum can be explained in terms of Langmuir waves generated by a ‘gap’ distribution formed through collisional losses over a timescale of several tens of minutes. Bursts are attributed to local enhancements in the Langmuir turbulence associated with a loss-cone instability. No triggering mechanism for the bursts is identified. It is predicted that if the continuum is due to a ‘large’ source then its brightness temperature should rise over several tens of minutes to a value which is roughly independent of frequency and of position across the source and which should not exceed 3 × 109 K. For bursts, it is predicted that a fainter second harmonic component should accompany bright bursts.