Abstract: An apparatus for simultaneously trimming both lateral marginal portions of a scalloped stream of sheets has a transporting unit which is driven to advance the stream in a horizontal cutting plane flanked by two stationary counterknives which are mounted below the cutting plane and whose cutting portions are disposed in the cutting plane. The sheets in the cutting plane are biased against the transporting unit and their marginal portions are trimmed by the cutting edges of radial knives mounted on two coaxial rotary holders disposed at a level above the cutting plane and each cooperating with a different one of the counterknives. The cutting edges of the knives are inclined with reference to the longitudinal directions of the respective knives and orbit along endless paths which are intersected by the cutting plane so that each cutting edge which reaches the cutting plane moves below such plane before beginning to move upwardly and above the cutting plane.