Abstract : The objective of the research is to determine the effects of mastery training and length of retention interval on retention of a procedural skill. Armor crewmen were individually trained to boresight and zero the main gun of the M60A1 tank. Crewmen were trained to either of two criteria: one correct performance (standard training) or three consecutive correct performances (mastery training). Crewmen's retention of the task was tested either one or five weeks after training. Each step of the task performance was scored GO or NO GO. When a crewman performed a step incorrectly, the scorer would correct the step before permitting the crewmen to continue. The results indicate a significant effect of both amount of training and length of retention interval on recall of the task, but no interaction between the variables. Crewmen perform better on the retention test after the shorter retention interval or after more intensive training. Differences in performance among the groups are mostly caused by differences on the first retention trial. There is no correlation between ability to perform or retain the task and mental category. The reason for this result may be the lack of variance among crewmen's mental categories.