The importance of vocabulary knowledge gained through incidental learning is well documented. The growth of incidental vocabulary knowledge is especially crucial for middle school students due to the complex words encountered in their studies. However, research on incidental vocabulary acquisition for middle school students is lacking. The purpose of this study was to compare the relative effectiveness of three instructional conditions (reading, writing, and reading and writing) on incidental vocabulary acquisition and retention with middle school students in an English as a first language (L1) environment. In this within subjects repeated measure study, 263 eighth-grade participants received treatment in three instructional conditions with three differing levels of involvement load. Data from 2,893 individual student measures were used to evaluate the influence of instructional conditions on incidental vocabulary acquisition and retention. Analysis of mixed-effects models showed that participant scores on the reading and writing condition were consistently higher than the writing only or the reading only condition. These results indicate that instructional tasks with higher involvement loads (e.g., reading and writing or writing) offer benefits to L1 middle school students for the incidental vocabulary acquisition necessary to be academically successful.