Magnetite nanometric powder was synthesized from metal salts using a coprecipitation technique. The powders were used to produce magnetic fluid via a peptization method, with hydrocarbon Isopar M as liquid carrier and oleic acid as surfactant. The complex magnetic susceptibility χ=χ′+iχ″ was measured as a function of temperature T in steps of 2.5 K from 3 to 298 K for frequencies ranging from f=10 to 10 000 Hz. The magnetic fluid real and imaginary components of the ac susceptibility show a prominent maximum at temperatures that increase with the measuring frequency, which is attributed to a spin-glass-like behavior. The peak temperature Tp1 of χ″ depends on f following the Vogel–Fulcher law f=f0 exp[E/kB(Tp1-T0)], where f0 and E are positive constants and T0 is a parameter related to particle interactions. There is another kind of peak temperature, Tp2, in the loss factor tan δ=χ″/χ′ which is related to a magnetic aftereffect. The peak temperature Tp2 is far less than Tp1 and shows an Arrhenius-type dependence on f.