We report a measurement of the fraction of b-quarks produced
diffractively in pbar p collisions at sqrt s = 1.8 TeV.  Diffraction is
identified by the absence of particles in a forward pseudorapidity region.
From events with an electron of transverse momentum 9.5 < p^e_T < 20 GeV/c
within the pseudorapidity region |eta| < 1.1, the ratio of diffractive to
total b-quark production rates is found to be R_bb = [0.62 +/- 0.19(stat) +/-
0.16(syst)]%.  This result is comparable in magnitude to corresponding ratios
for W and dijet production, but significantly lower than expectations based on
factorization.