  Using Korringa-Kohn-Rostoker coherent-potential approximation in the
atomic-sphere approximation (KKR-ASA CPA) method for taking into account the
effects of disorder, Gaspari-Gyorffy formalism for calculating the
electron-phonon coupling constant $\lambda$, and Allen-Dynes equation for
calculating T_c, we have studied the variation of T_c in Mg_{1-x}Ta_{x}B_2
alloys as a function of Ta) concentration. Our results show that the T_c
decreases with the addition of Ta for upto 40 at% and remains essentially zero
from 60 at% to 80 at% of Ta. We also find TaB_2 to be superconducting, albeit
at a lower temperature. Our analysis shows that the variation in T_c in
Mg_{1-x}Ta_{x}B_2 is mostly dictated by the changes in the B p density of
states with the addition of Ta.
