In this paper, we present a theoretical approach to calculate differential and total ionization cross sections of polyatomic molecules by fast electron impact. More exactly, we have studied the ionization of ammonia (NH3) and methane (CH4) molecules, and previous results concerning the H2O molecule ionization are reported for comparison. The calculations are performed in the distorted wave Born approximation without exchange by employing the independent electron model. The molecular target wave functions are described by linear combinations of atomic orbitals. To describe the interaction between the inactive target electrons and the slow ejected electron, we have introduced a distortion via an effective potential calculated for each molecular orbital. The present theoretical calculations agree well with a large set of existing experimental data in terms of multiple differential and total cross sections. (C) 2004 American Institute of Physics.