A Fabry-Perot-type interferometer is experimentally realized for electrons in a semiconductor device. Interference
conditions are created for co-propagating electrons in quantum Hall edge states, which results in oscillations of
the current through the device for integer and fractional filling factors. We find the interference oscillations
in transport across the incompressible strips with local filling factors <i>ν<sub>c</sub></i> = 1, 4/3, 2/3 even at high imbalances,
exceeding the spectral gaps. In contrast, there is no sign of the interference in transport across the principal
Laughlin <i>ν<sub>c</sub></i> = 1/3 incompressible strip. This indicates, that even at fractional <i>ν<sub>c</sub>, </i>the interference effects are caused by "normal" electrons. The oscillation's period is determined by the effective interferometer area, which is sensitive to the filling factors because of screening effects.