This publication summarizes the results of the joint utilization of the nuclear reactor `Yayoi` and the electron beam accelerator in the Nuclear Engineering Research Laboratory, University of Tokyo, in fiscal year 1991. The Yayoi was operated smoothly throughout the year, and the number of research themes, for which the reactor Yayoi was jointly utilized, and the related themes reached 21 cases. After the linear accelerator was reconstructed as the twin linac, the joint utilization was resumed in October, 1989, and the number of research themes, was 15 cases. In this publication, in addition to the utilization reports, also the reports of 15 cases of Yayoi Study Meetings held in fiscal year 1991 are collected. (K.I.).