  We present seismic analysis of the prototype SX Phoenicis that aims at
fitting the two radial-mode frequencies and the corresponding values of the
bolometric flux amplitude (the parameter $f$), whose empirical values are
derived from multi-coulor photometric observations. Seismic model that meets
these conditions is of low mass, $M=1.05 M_\odot$, has moderately effective
convection in the outer layers, described by the mixing length parameter
$\alpha_{\rm MLT} \approx 0.7$, and the microturbulent velocity in the
atmospheres of about $\xi_{\rm t}\approx 8$km/s. Such seismic studies of stars
like SX Phe are very important for deriving constraints on outer-layer
convection, because the object is on the border between very effective and
ineffective convection.
