A continuous rod-like filler is formed in a cigarette rod making machine on an endless foraminous belt conveyor by depositing a row of spaced-apart batches consisting of a first type of tobacco at one side of the conveyor, holding the batches by suction, and showering tobacco particles of a second type onto and/or into the spaces between the batches. The density of successive increments of the filler is monitored by a pneumatic and/or radiation type detector, and the thus obtained density signals are used to generate control signals which regulate the density of the filler as well as control signals which are used to influence the batches, if and when necessary. For this purpose, the density signals can be processed to single out those which are attributable to monitoring of the density of successive batches, and the singled out density signals are then analyzed and used to vary one or more parameters of the batches.