The experimental study of small signal dielectric nonlinearity is a powerful procedure for the investigation of complex ferroelectric materials. Dielectric nonlinearity of a copolymer of polyvinylidenefluoride with trifluoroethylene, in the compositions 70/30 and 56/44 mol%, has been measured by an analysis of the harmonic spectrum of the electric current in response to a sinusoidal voltage. The Landau parameters of the crystalline phase are determined from the experimental data, and the order of the phase transition is discussed. It is found that the values of the Landau parameter gamma in the ferroelectric and in the paraelectric phase are different. In the ferroelectric phase gamma is a function of the degree of poling. The experimental results are also compared with theoretical predictions derived from Odajima's microscopic model, which assumes a one-dimensional Ising model for the dipolar coupling along the polymer chains and a mean field theory for the interchain interaction. Copolymer film deposited from a solution that is poled before annealing shows a small second-order dielectric nonlinearity even in the paraelectric phase. The reason for this is a non-switchable polarization, which is stable above the Curie temperature. We attribute this polarization to the intermediate phase in between the crystallites and the amorphous regions.