The letter reports observations of the J = 1-0 and J = 8-7 rotational transitions of cyanodiacetylene (HC5N) at the respective frequencies of 2.663 GHz and 21.301 GHz in Sgr B2. A value of -4.12 + or 0.07 MHz for the HC5N quadrupole coupling constant is obtained from the resolved hyperfine splitting of the J = 1-0 transition. Assuming thermal equilibrium of the molecules at an excitation temperature of 30 K and an optical depth of much less than unity, a total column density of 1.5 by 10 to the 14th power per sq cm is computed for both the J = 8-7 transition and the previously observed J = 4-3 transition. An anomalously large intensity is found for the J = 1-0 transition in HC5N, suggesting maser action in this molecule. Additional theoretical evidence is cited in support of weak maser action in the J = 1-0 line of HC5N.