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We report a measurement of the fraction of b quarks produced diffractively in p̅p collisions at √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< pTe /c within the pseudorapidity region |η| < 1.1, the ratio of diffractive to total b-quark production rates is found to be Rbb= [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.