The study investigated the effect of instruction on basic number fact mastery of 123 learning disabled (LD) students, 8 to 13 years old. Mastery of basic addition, subtraction, and multiplication facts was seen as an important component of overall arithmetic competence and represents a particular area of performance deficit among LD students. The intervention study compared two treatment approaches containing parallel program features, but with differences in the sequential clusters of facts taught. Instruction on each cluster of facts involved four phases: (1) presentation of activities (during group lessons the number facts in the cluster were explored using concrete materials and/or graphic representations); (2) developmental activities (active practice of fact clusters emphasized accuracy without regard to speed with teachers supervising pairs or small groups of students playing one or more games, performing oral or blackboard reviews, or individuals working on activity sheets); (3) mastery activities (practice of fact clusters emphasizing rapid, automatic responses); and (4) criterion testing (students had to meet a preestablished criterion before moving from one cluster of facts to another). Basic Fact Sequence 1 followed traditional grouping and sequencing, while Basic Fact Sequence 2 grouped and ordered related facts according to "thinking" strategies. Results of pre-, post-, and retention tests, each 3-minute written power tests of basic fact proficiency were analyzed. Significant gains were made on post-tests; these gains were maintained during a 6-week uninstructed period. No effect was found for Basic Fact Sequences, leading to the conclusion that either sequence is effective in promoting mastery of basic facts under the instructional conditions utilized. (Author/SW)