Fatigue crack growth rates have been determined for a series of polyethylenes through the application of various periodic loading and unloading programs in which the load was held constant for f(ON) seconds and unloaded to a lower constant value for to, seconds in each cycle. The crack growth rate, da/dN, was given by da/dN = gamma t(ON)[1 + k(1 - exp(-(t(OFF)/beta t(ON))(n)))] where the parameters gamma, k, beta and n are material dependent and beta and n are also geometry dependent. It was found that beta and n described the closure of the crack during the unloaded period as well as the kinetics of the increase in crack growth rate. It is proposed that this equation is consistent with a mechanism in which the crack growth rate increases because of damage sustained by the craze during the unloaded period as it closes and is compressed under the recovery of the surrounding material.