Crack propagation of PMMA in some liquid environments is described for various testing conditions, such as fixed load, fixed displacement and monotonically increasing displacement. Fracture mechanics concepts have been used successfully in analysing the results. When continuous stable cracking is achieved, values of fracture toughness (R) for PMMA under these loading conditions are obtained as a function of crack velocity (ŀ) using the method of Gurney and Hunt [11]. For crack velocities greater than 10−2 mm sec−1, the fracture toughness values in the environments are increased when compared with the corresponding air results. Unique relationships betweenR andŀ have been shown to exist for cracking in ethanol and carbon tetrachloride.