In the emission spectrum of the carbon monoxide 13C16O isotopic molecule three bands comprising about 1820 lines of the 3A band system (c3Π-a3Π) were recorded and analyzed. The 0-0 and 0-1 bands of this system were photographed for the first time and the 0-2 band was rephotographed by using methods of conventional high-resolution spectroscopy. The result of the rotational band analysis includes expanding of the spectrum interpretation up to J = 25 as well as the identification of four previously unobserved branches P13, R13, P31, and R31. Because of strong perturbations in the c3Π (ν = 0) state, the calculation of the rovibronic structure constants was performed only for the lower a3Π state. By using a calculation based on a nonlinear least-squares method, an effective Hamiltonian of Brown [J. M. Brown, E. A. Colbourn, J. K. G. Watson, and F. D. Wayne, J. Mol. Spectrosc. 74, 294-318 (1979)] and a separative procedure proposed by Curl-Dane-Watson [R. F. Curl and C. B. Dane, J. Mol. Spectrosc. 128, 406-412 (1988); J. K. G. Watson, J. Mol. Spectrosc. 138, 302-308 (1989)], it was possible to derive the rotational structure constants for the ν = 0, 1, and 2 levels for the a3Π state in the 13C16O isotopic molecule. Term values for the C3Π (ν = 0) level and the equilibrium molecular constants for the a3Π state also are reported. © 1999 Academic Press.