This report uses data from the 1989-90 National Postsecondary Student Aid Study (NPSAS:90) to describe the population of part-time undergraduates who were enrolled in postsecondary education in the United States during the 1989-90 academic year. It examined: (1) how enrollment status varied with a range of student and attendance characteristics; (2) differences in the composition of the part-time and full-time student population; (3) the grades earned by part-time undergraduates and the length of time they were enrolled in 1989-90; and (4) tuition costs and receipt of financial aid among part-time undergraduates. The report found that from 1970 to 1990, the number of part-time undergraduates more than doubled while full-time enrollment increased by one-third. It also found that half of all undergraduates in postsecondary education in 1989-90 were enrolled full time throughout their enrollment during the year; 36 percent were enrolled exclusively part time; and the remaining 14 percent changed their enrollment status during the year. Other significant findings are presented. Two appendixes contain a glossary and a description of the NPSAS's methodology and statistical procedures. (MDM)