Magnetic ﬁeld-induced dispersion of ultrasonic velocity in a Mn0.7Zn0.3Fe2O4 ﬂuid (applied magnetic ﬁeld is perpendicular to the ultrasonic propagation vector) is determined by employing continuous wave method. The magnitude of dispersion initially decreases with increasing ﬁeld, then increases and reaches a plateau at higher ﬁelds. Results indicate that the velocity anisotropy is dominated by grain–grain interactions rather than grain–ﬁeld interaction. At the critical temperature, the grain–grain interaction becomes weak as the transverse component of the particle/cluster moment is larger than the longitudinal one and the system reaches saturation even at low ﬁeld. These observed variations in the ﬁeld-induced anisotropy are analysed by incorporating the moment distribution of particles in Tarapov’s theory (J. Magn. Magn. Mater. 39, 51 (1983)).