In the 1990s, SLA research began to present empirical evidence that form-focused-instruction (FFI) has long-lasting positive impacts on the acquisition of specific syntactic features, which are acquired in predictable sequences of interlanguage development. For example, three published studies reported the sustained effects of FFI on the acquisition of interrogatives in normal ESL classes with school-age children (Spada & Lightbown, 1993) and in experimental classes with adult ESL learners (Mackey, 1999; Mackey & Philp1998). These supported Pienemann's (1984) Teachability Hypothesis, which claims that learners can benefit from FFI if it targets the next stage of their L2 development rather than a stage further beyond the learner's current level. However, the previous FFI research has not explored the acquisition of question forms at the highest developmental stage (Stage 6), such as cancel-inversion questions, tag questions, and negative questions and little is understood about what types of intervention are effective in promoting adult learners' acquisition of these interrogatives. Moreover, few studies have been conducted in the EFL context, let alone, in intact EFL classrooms and little is known about the acquisition of interrogatives by Japanese learners of English, whose mother language applies very different word order from the target language. My research aims to investigate the issues raised above. A preliminary analysis of 31 Japanese university students' oral performances in the pre-, post- and follow-up tests given before and after four 90-minute teaching sessions will be reported in comparison with those produced by 12 students who voluntarily joined the three tests without the treatment.