Cucumber seedlings grown in microgravity developed a peg on each side of the transition zone (TR) between hypocotyl and root, whereas seedlings grown in a horizontal position on the ground developed a peg on the concave side of the gravitropically bending TR. The morphological features of the space-grown seedlings were similar to that of seedlings grown in a vertical position on the ground with their radicles pointing down; both became two-pegged seedlings. Morphogenesis of cucumber seedlings is thus negatively controlled by gravity. Analysis by in situ hybridization of an auxin-inducible gene, CS-IAA1, showed that its mRNA accumulated much more on the lower side of the TR in the horizontallyplaced seedlings on the ground just prior to and at the initiation period of peg formation. On the other hand, when seedlings were grown in microgravity or in a vertical position on the ground, accumulation of CS-IAA1 mRNA was seen all around the transition zone. It appeared that the accumulation of CS-IAA1 mRNA decreased on the upper side of the transition zone upon gravistimulation. Application of exogenous IAA to the seedlings in a horizontal position caused the development of a peg on each side of the transition zone, or a collar-like protuberance, depending on the concentration used. These results suggest that there is a threshold level of auxin concentration for peg formation in cucumber seedlings. Upon gravistimulation, auxin concentration on the upper side of the horizontally placed transition zone is possibly reduced to the level below the threshold value.