The importance of .successful usage of prepositions to the social, communicative, and academic achievement of the school age child has been documented in the literature. Information on language skills of educable mentally· retarded (EMR) children is of special concern because they represent the sub-group o~ mentally deficient individuals most likely to interact and participa'te in academic settings and community life.The purpose of this study was to investigate·which of 26 prepositions are acquired expressively by.educable mentally retarded children.This study was designed to 'determine i f there was a significant difference between the number of prepo.sitions expressed by an EMR sample and those expressed by normals of similar mental age, and if the same prepositions were used by the two groups at given mental ages. The correlation between chronological age and the number of prepositions expressed by the EMR population was also inves'tigated.2 Thirty-twQ subjects in two mental age groupings participated in this study. At mental age six years (± three months), eigh.t EMR subjects and eight normals were selected. Eight individu~ls also comprised both groups at mental· age eight years(± three months). The Hustead Expre~sive Preposition Test, (HEPT) (Hustead, 1978) was administered to all .subjects.Results showed no statistically significant difference between the EMR ana normal groups at.either mental age in the number of prepositions expressed, i.e., the EMR students general~y were found 'to express the ·same number of prepositio~s as normals of similar mental ages. A moderate correlation between chronological age and the numb~r of prepo-· sitions express~d by the EMR subjects was evident, suggesting experience and maturation could be related to the number of prepositions expressed; however, this relationship could be attributed in part to the older mental age of the children of older chronological age.Comparison of the prepositions used by both groups at mental ages six and eight years revealed more similarities· than dif fere~ces.Generally, the same prepo~itions were used by .the subjects in the control and EMR groups at each mental age. Prepositions were found to develop similarly in number and type for both groups. Results of this study thus tend to indicate a "developmental lag" in usage of prepositions rather than a qualitative or disordered development.3