This paper presents the results of a parallel classroom experiment investigating the effects of processing instruction, traditional instruction and meaning-based output instruction on the acquisition of the English past simple tense. The subjects involved in the present studies were Chinese and Greek school-age learners of English residing in their respective countries. The participants in both schools were divided into three groups. The first group received processing instruction; the second group was exposed to traditional instruction; the third group received meaning-based output instruction. One interpretation and one production measure were used in a pre-test and post-test design (immediate effect only). The results showed that processing instruction had positive effects on the processing and acquisition of the target feature. In both studies the processing instruction group performed better than the traditional instruction and meaning-based output instruction groups in the interpretation task and the three groups made equal gains in the production task. (Contains 1 note, 4 tables, and 1 figure.)