  A new SX Phe star (labelled SXP1) found from $BV$ CCD photometry is the first
to be discovered in the globular cluster M15. It is a blue straggler and is
located $102\arcsec.8$ north and $285\arcsec.6$ west of the center of M15
\citep{har96}. Mean magnitudes of SXP1 are $<$$B$$>$ = 18$\fm$671 and $<$$V$$>$
= 18$\fm$445. The amplitude of variability of SXP1 is measured to be $\Delta V
\approx 0.15$. From multiple-frequency analysis based on the Fourier
decomposition method, we detect two very closely separated pulsating
frequencies: the primary frequency at $f_1=24.630$ c/d for both $B$- and
$V$-bands, and the secondary frequency at $f_2=24.338$ c/d for the $B$-band and
24.343 c/d for the $V$-band. This star is the second among known SX Phe stars
found to pulsate with very closely separated frequencies ($f_2/f_1\ge0.95$).
These frequencies may be explained by excitation of nonradial modes; however,
we have an incomplete understanding of this phenomenon in the case of SX Phe
stars with relatively high amplitudes. The metallicity-period and the
variability amplitude-period relations for SXP1 in M15 are found to be
consistent with those for SX Phe stars in other globular clusters.
