We report an experimental study of coherent population trapping and four-wave mixing via multilevel dark states prepared in the ground hyperfine states of 87Rb atoms. The dark states are created by A-type coupling chains driven by a linearly polarized pump laser tuned to the D1 F=2↔F′=1 transition. The Doppler shift in the coupling chains is compensated by the left- and right-circularly polarized components of the pump laser. The multilevel dark states are explored by a probe laser and their existence is manifested by a strong dependence of the probe excitation and four-wave-mixing emission on the probe polarization. Our experiment shows that coherent population trapping in multilevel dark states of a degenerate atomic system should be useful in the coherence control of optical nonlinearity.