Ionic ZnFe2O4 is a weak magnetic ferrofluid with different particle sizes (a polydisperse ferrofluid), in which the dipolar coupling constant λ is less than 2. By comparing the reduced magnetization curves and initial magnetic susceptibilities of ferrofluids with different particle volume fractions ϕ, it is found that there are field-induced attractive interactions between the colloidal particles, which make them aggregate; this interaction decreases with ϕ. The dipolar coupling constant is so small that particle aggregation cannot be induced by magnetic interaction alone. Thus, it can be concluded from the experimental results that a nonmagnetic attractive interaction is stimulated during the magnetization process for ferrofluids. Using a model based on “large” and “small” particles (the bidisperse model), this nonmagnetic interaction can be explained as a field-induced structural force. This force is enhanced following magnetic interactions between the large particles as the applied magnetic field increases. The field-induced structural force is similar to a depletion force and can also arise in strong magnetic ferrofluids (λ>2) with polydisperse particle sizes under an applied external magnetic field.