  We have uncovered a new class of small amplitude magnetoresistance
oscillation in unidirectional lateral superlattice (ULSL). The oscillation is
observed in a low-field regime, typically |B| =< 0.03 T, as small undulation on
top of well-known positive magnetoresistance background. Positions of maxima of
the oscillation shift to lower field side with the increase of the electron
concentration n_e roughly proportionally to n_e^{-1/2}, and also with the
increase of period a of ULSL samples. The oscillation is attributed to
commensurability between the period a and the width of open orbits originating
from the miniband structure.
