Interstellar C-3 toward HD 210121

We report the detection of the 405 nm band of interstellar C-3 in absorption toward HD 210121. The abundance of triatomic carbon is approximately 1/17 of that of diatomic carbon in the same diffuse molecular cloud. Rotational levels of C-3 up to J = 14 are seen in this cloud. The rotational excitation of C-3 in the interstellar medium may reflect a competition between inelastic collisions, formation and destruction of the molecule, and radiative pumping in the far-infrared. The abundance of C-3 is compared with chemical models. Attention is called to molecular properties that need to be better determined.