An electronic musical instrument automatically plays a musical accompaniment utilizing a minimized number of tone signal generating channels. The electronic musical instrument has a plurality of tone signal generating channels each for generating a tone signal representative of a tone to be produced. Keys depressed on an associated keyboard are assigned to the tone signal generating channels to cause them to generate tone signals corresponding to the depressed keys. When an automatic accompaniment circuit is actuated, a predetermined number of ones of the plurality of tone signal generating channels are reserved for generation of tone signals of the accompaniment. The reservation is made such that those channels which are idle are selected at the highest priority, those channels to which the earliest released keys are assigned at the second highest priority, and those channels to which the earliest depressed keys are assigned at the third highest priority.