Missing -ing in reading: Developmental changes in reading units

College students and school children between the ages of 6 and 10 were asked to search four 50-word passages for instances of the target letter n. The subjects made most letter detection errors on the function words in and and, and on the suffix morpheme -ing. This “missing letter” effect showed a developmental trend, with the proportion of errors on function words and suffix morphemes increasing as a function of school grade. The data are consistent with the hypothesis that frequent linguistic entities such as word-boundary morphemes tend to be unitized in fluent reading, and that the ability to form such units is acquired gradually in the course of learning to read.