The propagation of ultrasonic waves in iron‐in‐mercury magnetic fluids has been studied by a continuous wave (CW) method. The CW method proved to be adequate for studying the velocity change (&Dgr;V) due to an external magnetic field. The magnitude of &Dgr;V was about 0.3% for field of 100 G. &Dgr;V was also measured at two field angles, when the direction of ultrasonic propagation is either perpendicular or parallel to the magnetic field directions. The characteristic time was found to be about 60 sec, which is of the same order as found from other magnetic measurements in the same sample. This fact indicated that there are aggregates of the size 104A˚ in the fluid.