A study was conducted to assess the effect of contrasting mastery learning procedures on the reading achievement of high and low ability students. Subjects were 48 high and 40 low ability first grade students randomly assigned to either a typical commercial basal reading series mastery treatment or an alternative mastery learning treatment that adhered more closely to mastery principles of frequent testing, corrective feedback, and technically sound measurement. Subjects were pretested using the "Passage Reading Test" (PRT) and posttested using the same measure at the end of the mastery treatment. The results indicated that the reading achievement of high ability students was not affected by the type of mastery learning method, whereas the reading achievement of low ability students was improved by the alternative mastery method. (HTH)