  We show that FCNC processes are unavoidable for leptoquarks that couple to
left handed quarks, and derive new FCNC bounds from neutral meson mixing.
Despite being induced only at one loop, these processes lead to significant
bounds since the leptoquark contributions do not suffer from GIM cancellations.
Studying the implications of these bounds we find that (i) The D-Dbar mixing
bound is the first significant FCNC bound from the up sector. Combining it with
FCNC bounds from the down sector we arrive at a bound on the first generation
couplings. (ii) The K-Kbar and D-Dbar mixings bound g^2/M while all other
processes bound g/M. The combined neutral meson mixing bound is therefore
dominant for the heavier leptoquarks, and leads to exclusion of large regions
in parameter space which were previously allowed.
