To isolate the process deficits underlying a specific learning disability in mathematics achievement, 77 academically normal and 46 learning disabled (LD) students in second, fourth or sixth grade were presented 140 simple addition problems using a true-false reaction time verification paradigm. (The problems were on a video screen controlled by an Apple II+ computer.) Regression models indicated that the majority of normal and LD second grade subjects used an implicit counting strategy for problem solution. However, LD subjects required a greater amount of time to execute this task and appeared to be deficient in the ability to self-monitor the problem solving process. A shift from reliance on the counting strategy to a memory retrieval process from the second to sixth grade was evident for normal subjects, but not for LD subjects, the majority of whom relied on the counting strategy throughout the elementary school years. (Author/CL)