ESR measurements have been made on a newly synthesized cocrystallite of C60 with the CoII complex of 5, 10, 15, 20-tetrakis[3, 5-(di-tert-butyl)-phenyl]porphyrin (tbp)-single crystal to study the strong intermolecular interactions which are suggested by x-ray structural analyses. A single anisotropic ESR absorption line is observed at room temperature with the principal g-values g⊥ = 2.423 and g∥= 2.005, which are typical values for the low-spin CoII ions (S = 1/2). The antiferromagnetic superexchange interaction between CoII ions via C60 is confirmed from the observation of the finite Weiss temperature, which is further supported by the absence of the hyperfine splitting due to 59Co (I = 7/2) nuclei. As the temperature is lowered, the g⊥-value decreases monotonically suggesting an increase of the crystal-field splitting energy. This result is consistent with the fact that the intermolecular distance between CoII and C60 becomes shorter as the temperature decreases. The peak-to-peak ESR linewidths ΔHpp along both principal axes increase with temperature due to the shorter spin-lattice relaxation time.