The cyclic C₃H radical (c-C₃H) was observed first in the laboratory and then toward TMC-1 from Nobeyama Radio Observatory by Yamamoto et al. in 1987. Mangum & Wootten observed c-C₃H in 13 additional sources. A theoretical study by Ochsenfeld et al. concluded that c-C₃H could be formed efficiently in the interstellar medium through the reaction of carbon atoms with acetylene.

c-C₃H is a radical of cyclopropenylidene and is therefore also a carbene, where the bridging carbon only forms two single bonds with the other two carbons.