The present study examined individual differences in strategy use in the context of participants' use and execution of a heuristic for verifying mathematical equations, the five rule (i.e., reject equations that contain a 5 as a multiplicand and that hare neither a 5 nor a 0 as the final digit of the proposed answer). The proportion, of problems that violated this rule varied by block. Tendency to adopt the heuristic was inferred from the size of the latency and accuracy advantage for the fire rule violations as compared with the false problems. Participants of varying skill let els differ in the speed and accuracy of executing the five-rule checking strategy as well as in the adaptivity of their strategy choices. Moreover, both low- and high-arithmetic skill participants used the five-rule on large problems only. The findings hare a number of implications for understanding which variables affect strategy use and for understanding individual differences in strategy use.