The project reported in this paper aims to test the concept of "learner developmental readiness" and its pedagogic effectiveness in the teaching of foreign language grammar. It focuses on the teaching of English as a second language (ESL) in a formal classroom context. The aim is to ascertain whether a specific teaching order based on the concept of developmental readiness, can enhance learning outcomes in foreign language classrooms. The main theoretical approach used is the Teachability Hypothesis articulated in Pienemann's (1998) Processability Theory (PT), which "predicts that stages of acquisition cannot be skipped through formal instruction and that instruction will be beneficial if it focuses on structures from "the next stage"" (Pienemann, 1998, p.13). Past teachability studies (e.g. Boss, 1996; Dyson, 1996; Ellis, 1989; Pienemann, 1984; and Spada & Lightbown, 1999) have employed predicted order testing. However in this study subjects were exposed to English syntax structures either in the predicted or in the reversed orders outlined under PT. The findings of this study show that learners exposed to instruction in accordance with the developmental order predicted in PT produce the target language (TL) structures with a higher grammatical accuracy than those exposed to the reversed order. This suggests that instruction is more beneficial, in relation to grammatical accuracy, when it focuses on the TL structures in a developmentally implicational manner. (Contains 8 tables, 2 figures and 1 note.)