Four major points summarize the findings of studies for Category IV (low ability) Marines and sailors. First, Armed Forces Qualifying Test (AFQT) score is a valid predictor of an enlistee's performance and adjustment in military service. Category IV enlistees are inferior to average enlistees on a variety of measures of military adaptation. Second, many Category IV enlistees can render effective military service, on the basis of sampling of higher level Category IV personnel who entered the service in 1960 and 1961. Seventy-three percent of Marines and 65% of sailors were found to be effective performers. Third, civilian educational achievement was not only uniquely related to four-year military effectiveness for both services, but was also the most important of the biographical predictors studied. Fourth, it was hypothesized that differences in effectiveness for Category IV enlistees who entered the Navy and Marine Corps in 1960 and 1961 were due more to differing personnel and administrative policies than to the caliber of enlistees. (authors/ly)