ABSTRACTComprehension and strategy use by autistic children were tested in two experiments. Eighteen autistic subjects were compared with thirty normal 3- and 4-year-olds, matched on the PPVT and Raven's Colored Progressive Matrices. In Experiment I subjects were asked to act out active and passive, biased and reversible sentences. The autistic group's overall comprehension was lower than that of the normal controls; although the autistic subjects did use a word-order strategy, they did not generally use a probable-event strategy. These findings were confirmed in Experiment II, in which the same procedure with anomalous three-word items was used. The results are interpreted as evidence that in autism there is a semantic-cognitive deficit in utilizing conceptual knowledge about relational aspects of the environment and that this deficit underlies the comprehension difficulties of autistic persons.