  We present results from 16 snapshots of Aql X-1 with RXTE during the rising
phase of its recent outburst. The observations were carried out at a typical
rate of once or twice per day. The source shows interesting spectral evolution
during this period. Phenomenologically, it bears remarkable similarities to
``atoll'' sources. Shortly after the onset of the outburst, the source is seen
to be in an ``island'' state, but with little X-ray variability. It then
appears to have made a rapid spectral transition (on a time scale less than
half a day) to another ``island'' state, where it evolves slightly and stays
for 4 days. In this state, the observed X-ray flux becomes increasingly
variable as the source brightens. Quasi-period oscillation (QPO) in the X-ray
intensity is detected in the frequency range 670--870 Hz. The QPO frequency
increases with the X-ray flux while its fractional rms decreases. The QPO
becomes undetectable following a transition to a ``banana'' state, where the
source continues its evolution by moving up and down the ``banana'' branch in
the color-color diagram as the flux (presumably, the mass accretion rate)
fluctuates around the peak of the outburst. Throughout the entire period, the
power density spectrum is dominated by very-low frequency noises. Little power
can be seen above ~1 Hz, which is different from typical ``atoll'' sources. In
the ``banana'' state, the overall X-ray variability remains low (with
fractional rms ~3--4%) but roughly constant. The observed X-ray spectrum is
soft with few photons from above $\sim$25 keV, implying the thermal origin of
the emission. The evolution of both spectral and temporal X-ray properties is
discussed in the context of disk-instability models.
