Instabilities of a ballooning type were observed in the ∮dl/Bstable region of a linear quadrupole device. The amplitude distribution of the fluctuation along a field line behaved like a standing wave that had a node where ▿B2· ▿p< 0 and was peaked where ▿B2· ▿p> 0. The quantityk∥&lgr;mfpewas typically 5–10. The observed fluctuation is in qualitative agreement with the theory of collisionless drift waves modified to take into account the change of field curvature and field strength along the field line. The threshold for the instability appears to be strongly dependent on the ratio of the plasma radius to connection length. The plasma confinement was not sensitive to the presence of the ballooning instability.