The purpose of this study was to examine the role of vocabulary in writing across three genres. Fifth graders (N = 105) wrote three compositions: story, persuasive, and informative. Each composition revolved around the topic of outer space to control for background knowledge. Written compositions were scored for holistic writing quality and several different vocabulary constructs: diversity, maturity, elaboration, academic words, content words, and register. The results indicated that students vary their vocabulary usage by genre. Story text had higher diversity than informative text as well as higher maturity as compared to persuasive text. Persuasive text contained higher diversity than informative text, and higher register than both of the other genres. Informative text included more content words and elaboration than the other text types as well as more maturity than persuasive text. Additionally, multiple regression and commonality analysis indicated that the vocabulary constructs related to writing quality differed by genre. For story text, vocabulary diversity was a unique predictor, while for persuasive text, content words and register were unique predictors. Finally, for informative text content words was the strongest unique predictor explaining almost all of the total variance in the five factor model, although maturity was also a unique predictor.