Cross sections for the production of C11 in C targets were measured using C12 ions with energies of 0.40, 1.05, and 2.1 GeV/nucleon. These measurements were undertaken for the purpose of establishing primary standards for use as beam monitors. Annihilation radiation from C11(T12=20.34 min) was counted using a large volume NaI detector. Cross sections of 63.5±0.5, 57.4±0.4, and 60.9±0.6 mb, respectively, were obtained for 0.40, 1.05, and 2.1 GeV/nucleon C12 beams. The results are compared with earlier measurements of the cross sections in C targets using relativistic proton and α beams. Good agreement is found between the cross sections measured in this work and a simple Glauber theory. NUCLEAR REACTIONS C12(C12, X)C11: E(C12)=0.40, 1.05, and 2.1 GeV/nucleon; measured σ.