The effect of plasticizer concentration on fatigue crack propagation (FCP) rate in cellulose acetate-propionate (CAP) was determined. Compact tension specimens were machined from 6.2 mm-thick injection molded plaques and tested on an MTS servohydraulic testing machine using a sinusoidal waveform with a frequency of 1 Hz. Two FCP mechanisms were identified : a crazing mechanism, which dominated at low values of stress intensity factor range, DELTAK, and a shear yielding mechanism, which dominated at high values of DELTAK. The value of DELTAK at the onset of the transition from the crazing mechanism to the shear yielding mechanism was a function of plasticizer concentration, and therefore yield strength of the CAP. The transition in crack propagation mechanism created a V-shaped feature on the fracture surface, which could be used to weight the contributions from the two crack propagation mechanisms to the overall FCP rate.