  We note that the recent direct measurement of the top quark mass at $173.3
\pm 5.6 (stat) \pm 6.2 (syst)$ by D0 collaboration severely constrains the
theoretically attractive infra-red fixed point scenario of the top quark Yukawa
coupling in supersymmetric GUTs. For one-step unified models the above
mentioned measurement bounds the arbitrary but experimentally determinable
parameter $\tan \beta$ to the range $1.3 \le \tan \beta \le 2.1$. Further
crunch on the top quark mass may determine $\tan \beta$ even more accurately
within the fixed point scenario. On the other hand an experimental value of
$\tan \beta > 2.1$ will rule out the fixed point scenario bounding
$h^2_t(M_X)/4 \pi$ to 0.022 from above.
