We report the first measurement of the exclusive decay
B0(s) --> D_s D+_s using 355 pb-1 of data collected by the CDF
II detector in p anti-p collisions at s**(1/2) = 1.96 TeV at the
Fermilab Tevatron. We measure the relative branching ratio B(B0(s)
--> D(s)- D+_s)/B(B0 --> D0 --> D-D+_s) = 1.44+0.48_-0.44.
Using the world average value for B(B0 --> D- D+_s), we find B(B0(s)
--> D(s)- D+_s) = (9.4+4.4_-4.2) x 10-3. This provides a lower
bound Delta GammaCP_s/Gamma_s > 2B(B0(s) --> D(s)- D+_s) >
1.2 x 10-2 at 95% C.L.