A device for applying a foamed treating material under pressure to a traveling sheet of relatively incompressible textile yarns. A foam applicator unit has a nozzle with a foam dispensing slot facing across one side of the traveling sheet of yarns. A drive roll faces the other side of the traveling sheet of yarns in tangential alignment with the foam dispensing slot. The roll has a resiliently compressible, soft rubber outer layer, which compresses to conform with the surface of the relatively incompressible yarns and presses the traveling sheet of yarns against the applicator surfaces leading to and away from the slot to prevent escape of foam and to prevent passage of foam between yarns to maintain uniform distribution of foam applied to the traveling sheet of yarns.