This study investigated the cognitive effort of 5th and 9th graders while writing a text. We manipulated genre (narrative text vs. argumentative text) and tested how level of handwriting automatisation contributes to cognitive effort and fluency in writing. The participants were 23 students from Grade 5 and 21 from Grade 9, who wrote two texts differing in genre while performing a secondary reaction time task. The results showed that cognitive effort interacted with genre. Cognitive effort decreased between Grades 5 and 9 only for writing argumentative text. Handwriting did not contribute to fluency in writing, but contributed to cognitive effort only in 5th-graders' writing of narrative text. The findings are discussed in light of the factors contributing to cognitive effort and fluency in writing. (Contains 1 figure and 1 table.)