The role of input and output in the acquisition of language has been a source of controversy in Second Language Acquisition (SLA) research. This present study aimed to investigate the relative effects of processing instruction (PI) as a type of input-based instruction and traditional instruction (TI) as a type of output-based instruction. Specifically, this experiment examined whether PI and TI bring about any improvement in comprehension and production of the Arabic subjunctive by beginner-level learners of Arabic. The PI instructional technique was based on the principles of input processing suggested by VanPatten (1993, 2002, 2004). It has three main elements: (a) an explicit explanation of grammar, (b) information on processing strategies, and (c) structured input activities. The study involved second semester students of Arabic and it aimed at assessing the impact of PI and traditional output instruction on the interpretation and production of the Arabic subjunctive on immediate and delayed posttests. One instructional package was developed for the PI group and another package was developed for the TI group. To assess the effects of instruction, a pretest/posttest/delayed posttest procedure with three tests was used. Each test included: 1) interpretation task with sixteen multiple choice items and 2) production task with sixteen sentence-completion items. The results from this study showed that participants who received PI outperformed participants from the TI as measured by Interpretation tasks of the subjunctive. However, the performance of both groups were statistically similar as was measured by the production tasks of the subjunctive. These results supported those of previous research that had compared PI with TI (Benati, 2001, 2005; Cadierno, 1995; VanPatten & Cadierno, 1993a, 1993b; VanPatten & Wong, 2004).