In light of the increasing enrollment of part-time students at public two-year colleges, the question arises whether the conventional ratio (3:1 or so) for converting part-time to full-time equivalent (FTE) enrollment accurately represents the actual costs of providing services to part-time versus full-time students. A study was conducted to assess the relative effect of part-time versus full-time students on several types of expenditures in the two-year college. Using a translog model, costs were represented by reported expenditures; output by the number of full-time, part-time, and non-credit students; prices by average salaries for full-time faculty; and technological conditions by program emphasis, the percent of students earning a degree, and the system status of the institution. The study found that: (1) the marginal costs for instruction at small colleges were estimated to be $1335 for a full-time student and $245 for a part-time student, while costs at large institutions were estimated to be $1941 for a full-time student and $194 for a part-time student; (2) regardless of institutional size, the ratio of full-time to part-time marginal costs was greater in the instructional area than in student services; and (3) at the majority of the two-year colleges in the sample it cost only about one-fifth to one-seventh as much to provide instructional services for a part-time student as a full-time student. A five-page list of references and cost data for the 779 colleges in the study sample are appended. (EJV)