Abstract A comprehensive description is given of the large 4N/X2.3 flare of 1981 April 1 on the basis of observed data in the x-ray, optical (Hα and white-light) and radio-ranges. Our analysis shows the following: The disappearance and contraction of the bright bridges of the large sunspots (as a manifestation of changes in magnetic flux or an EMF), the disappearance and re-appearance of sunspots and the motion of the small sunspots could be the direct cause of this burst. The increase in the curvature of the dark filament before the flare indicated a compression of the magnetic field or an increase in the shear. The soft and hard x-ray burst sources and the microwave source were located on the top of the magnetic loop. The non-thermal electrons with energies above 20 keV provided an energy of about 4.3 (+31) ergs within the first burst peak.