Drosophila melanogaster is able to perform osmotropotaxis under open-loop conditions. With an ‘optimal’ stimulus the average turning tendency to the side of higher concentration corresponds to a circular track with radiusr=0.8 cm. The response amplitude does not decrease within 1 or 2 h. Unilaterally antennectomized flies in an homogeneous odor field show a permanent turning tendency towards their intact side. The smallest concentration ratio to elicit osmotropotaxis in normal flies ranges between 6∶10 and 9∶10 at high and between 2∶10 and 5∶10 at an about 50 times lower odor intensity. No negative tropotaxis (i.e. turning to the side of lower concentration) is observed, even with strong repellents.