SUMMARYThe aim was to study the development of children's lexicogrammatical knowledge of words, in particular, their knowledge of the less frequent grammatical usages of words that may occur in more than one part of speech. To develop lists of such words, the grammatical functions of 1220 common words drawn from two word‐counts were examined; about 50 percent were found to be grammatically ambiguous. Data were collected from about 1500 children in grades 3, 6, and 9 to determine in what parts of speech 240 grammatically ambiguous words would be used when the children were asked to write sentences illustrating their uses; about 55 percent of these words were found to be used “infrequently” (according to a certain criterion) in one or more of their possible parts of speech. An intensive study was made of the comprehension, by 2000 3rd, 6th, and 9th grade children, of 65 words with infrequently used grammatical functions. For about 90 percent of these words, it was found that the children had significantly more difficulty in comprehending the infrequent grammatical functions than in comprehending the more usual grammatical functions. In many cases, grammatical function per se was a significant factor; in other cases, differential meanings of the words may also have been a factor. Developmental trends were noted, and it was concluded that acquisition of lexicogrammatical information about grammatically ambiguous words is a slow process that is far from complete even at the 9th grade level. Development of this knowledge is moderately well correlated with general vocabulary knowledge. Because lack of lexicogrammatical information is an important (and generally unrecognized) factor in comprehension difficulties, it is recommended that the English language arts curriculum pay greater attention to the explicit teaching of the less frequent grammatical functions of grammatically ambiguous words. The psycholinguistic implications of the results are discussed.