  We have analyzed seventeen observations of the low-mass X-ray binary and
atoll source 4U 1728-34, carried out by the Rossi X-ray Timing Explorer in 1996
and 1997. We obtain precise measurements of the frequencies of the two
simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source.
We show that the frequency separation between the two QPO, $\Delta \nu$, is
always significantly smaller than the frequency of the nearly-coherent
oscillations seen in this source during X-ray bursts, even at the lowest
inferred mass accretion rate, when $\Delta \nu$ seems to reach its maximum
value. We also find that $\Delta \nu$ decreases significantly, from $349.3 \pm
1.7$ Hz to $278.7 \pm 11.6$ Hz, as the frequency of the lower frequency kHz QPO
increases from 615 to 895 Hz. This is the first time that variations of the kHz
QPO peak separation are measured in a source which shows nearly-coherent
oscillations during bursts.
