We calculate supernova electron capture and beta-decay rates for various pf
-shell nuclei using large-scale shell-model techniques. We show that the ce
ntroid of the Gamow-Teller strength distribution was systematically misplac
ed in previous rate estimates. Our total electron capture rates are signifi
cantly smaller than currently adopted in core collapse calculations, while
the total beta-decay rates change less. Our calculation shows that for elec
tron-to-baryon ratios, Y-e = 0.42-0.46, beta-decay rates are larger than th
e competing electron capture rates.