To help assess the writing ability of college freshmen studying English as a second language (ESL), this study was undertaken to identify particular structures of subordination associated with writing maturity and to develop a diagnostic instrument to test student control of those structures. Following sentence-generating principles of transformational grammar, the developed examination, entitled Test of Ability to Subordinate (TAS), offers test items in the form of pairs or triads of kernel or "core" sentences which students are asked to combine into one sentence by filling in missing words in a given sentence frame. The 50-item, limited-response examination was administered to a number of ESL college freshmen along with the Michigan Test of English Language Proficiency and a composition exercise. The TAS correlated .86 with scores on the Michigan Test and .74 with composition scores. The study suggests that: (l) certain structures of subordination appear to be critical elements of overall writing ability; (2) it is possible to construct valid objective tests which ask students to actively engage in writing sentences as well as in a cognitive process required in free writing; and (3) the ability to produce certain transformations through sentence combining is indicative of the ability to perform these transformations during the normal writing process. (Author)