Writing samples representing argumentation, exposition, narration, and description were gathered from 51 fifth-grade pupils. Significant differences were found among the four modes of writing and among high-, medium-, and low-ability groups for T-unit length, clauses per T-unit, and clause length. Argumentation produced the most complex writing, and description produced the least complex, with exposition and narration sharing an intermediary position which varied according to the measure chosen. Reading comprehension correlated positively with T-unit length and clause length. Implications are discussed in terms of writing instruction, writing research, and education in general. (AA)