The present invention relates to a modular structure of apparatus (10) for treating fluids with ultrasound, of the type comprising at least one tubular element (11) defining a duct for a fluid being treated, there being, integrally fixed to said at least one tubular element (11), at least one supporting body (15) for a plurality of generators of ultrasonic vibrations (17, 18) for vibrating said at least one tubular element (11), the latter being connected to said supporting body (15) so as to allow the transmission of the vibrations for the generation of ultrasonic cavitation in the fluid being treated. Said structure of apparatus (10) comprises at least two tubular elements (11, 12), parallel and placed side by side, and fixed to at least a same supporting body (15), each segment (11a) of tubular element (11) fixed to a corresponding supporting body (15) being surrounded by at least two generators of ultrasonic vibrations (17, 18).
