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Results of observations of the Galactic bulge X-ray source GX 9+9 by theAll-Sky Monitor (ASM) and Proportional Counter Array (PCA) onboard the RossiX-ray Timing Explorer are presented. The ASM results show that the 4.19 hourX-ray periodicity first reported by Hertz and Wood in 1987 was weak or notdetected for most of the mission prior to late 2004, but then became strong andremained strong for approximately 2 years after which it weakened considerably.When the modulation at the 4.19 hour period is strong, it appears in foldedlight curves as an intensity dip over slightly less than 30% of a cycle and isdistinctly nonsinusoidal. A number of PCA observations of GX 9+9 were performedbefore the appearance of strong modulation; two were performed in 2006 duringthe epoch of strong modulation. Data obtained from the earlier PCA observationsyield at best limited evidence of the presence of phase-dependent intensitychanges, while the data from the later observations confirm the presence offlux minima with depths and phases compatible with those apparent in folded ASMlight curves. Light curves from a Chandra observation of GX 9+9 performed inthe year 2000 prior to the start of strong modulation show the possiblepresence of shallow dips at the predicted times. Optical observations performedin 2006 while the X-ray modulation was strong do not show an increase in thedegree of modulation at the 4.19 hour period. Implications of the changes inmodulation strength in X-rays and other observational results are considered.

GX 9+9: Variability of the X-Ray Orbital Modulation