The results of an ab initio study of the magnetic hyperfine structure in the X(2)Pi electronic state of CCCH are reported. The potential surfaces for two components of the X(2)Pi electronic state were computed by means of an extensive configuration interaction approach. The electronically averaged hyperfine coupling constants of H and C-13 for C-12 C-12 (CH)-C-12, C-13 C-12 (CH)-C-12, C-12 C-13 (CH)-C-12, and C-12 C-12 (CH)-C-13 are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The model Hamiltonian is expressed in terms of the normal bending coordinates. It is found that, due to the generally strong geometry dependence of the hyperfine coupling constants, it is necessary to carry out the vibronic averaging of the corresponding functions in order to obtain the values which can be compared to the results of the measurements. The results of the present study help to reliably interpret the experimental data previously published. They also predict the yet unobserved hyperfine structure in excited vibronic states. (C) 2004 American Institute of Physics.