The field of second language (L2) acquisition research has shown that for some grammatical structures in English, there seems to be an order of acquisition. (See for example, Dulay and Burt, 1974; Bailey, Madden and Krashen, 1974; Larsen-Freeman, 1976; Krashen, 1981.) Certain early acquired structures have been identified (e.g., -ing) as well as certain late acquired items (e.g., third person -s). Research has also shown, however, that it is not enough to examine orders of acquisition: one must also look at what are called 'transitional stages.' Accordingly, it is not sufficient to say that in English L2 learners' negation is acquired at X point in relation to other structures, for this does little to reveal what steps the learner took to arrive at the acquisition of that form. Closer looks at transitional stages have shown that learners of English as an L2 go through certain steps regardless of first language (L1) background and regardless of rate (see Dulay, Burt and Krashen, 1982, for a summary of English L2 negation development and a discussion of transitional stages). It would seem appropriate, then, to ask whether adult foreign language learners, even though in a formal situation, advance through certain stages of development regardless of level, L1, text, syllabus, and teacher. This paper is a first step toward that goal in that it reports on several stages of development in the acquisition of the Spanish copulas ser and estar in adult learners of Spanish. The question as to how the data in this study relate to current hypotheses about L2 acquisition is also raised. An important task of those who research foreign language (FL) learners as opposed to L2 learners (e.g., ESL learners) is to see if FL originated data can be explained or interpreted by the so-called 'general' principles of L2 acquisition research. In this manner, data from FL learners can help to confirm, refine, or even reject hypotheses based on research using L2 learners.