Laboratory detection of four isomers of C 5 H 2 molecule have been reported by Gottlieb et al. [Gottlieb, C.A., McCarthy, M.C., Gordon, V.D., Chakan, J.M., Apponi, A.J., Thaddeus, P., 1998. ApJ 509, L141]. Two transitions 3 13 −2 12 and 3 03 −2 02 at 19.147 and 19.606GHz, respectively, of c-C 5 H 2 (isomer 1) have been detected in TMC-1 by Dickens et al. [Dickens, J.E., Langer, W.D., Velusamy, T., 2001. ApJ 558, 693]. Chandra and Shinde [Chandra, S., Shinde, S.A., 2004. A&AS 423, 325] suggested that the c-C 5 H 2 may be identified in cool cosmic objects through its transition 3 13 −4 04 at 4.3GHz in absorption against the cosmic microwave background. Here, we have investigated the third isomer of C 5 H 2 . A number of lines 4 14 −5 05 , 3 13 −4 04 , 2 12 −3 03 , 6 15 −6 06 and 5 14 −5 05 of third isomer have been found in absorption against the CMB. We propose that detection of 4 14 −5 05 and 3 13 −4 04 transitions at 4.8 and 63.9GHz, respectively, of this isomer against the CMB may be used for identification of C 5 H 2 . Since in absence of availability of the collisional rates, we have used scaled values for them, we have checked the sensitivity of the lines on the rates by enhancing the rates for the transitions with Δk a =0 by a factor of 10. The results are not found sensitive.