The study evaluated the effect of a twelve-week program of conversational language instruction on the vocabulary development of four-year-old students in preschool classes. It also ascertained teachers' perceptions of the program. A mixed methods approach was used to answer the following research questions: (1) Is there a significant difference between the vocabulary scores of four-year-old children who received oral language instruction through a conversational classroom program and those who did not, as measured by the Peabody Picture Vocabulary Test, Fourth Edition? (2) What are the perceptions of teachers regarding the program, immediately after they receive training in its use, and after they implement it for twelve weeks with their students, as ascertained by questionnaires and interviews? The study was conducted in two private preschools in a suburb of a large southwestern city. There were 37 four year olds in three control classes in one school, and 42 four year olds in three experimental classes in other school. The Peabody Picture Vocabulary Tests, Form A and Form B, were used as pretest and posttest for both groups.

Before beginning the twelve-week treatment, the teachers of the experimental classes were trained in using conversation as a tool for vocabulary development of four year olds. These teachers provided questionnaire data and interview data immediately after the training sessions and again after the twelve-week implementation of the program in their classrooms.

In answer to the first research question, the statistical analysis of the data showed that there was a significant difference between the vocabulary scores of four year olds who received oral language instruction through the conversational classroom program and those who did not, as measured by the Peabody Picture Vocabulary Test. The posttest scores of the experimental group were significantly higher than those of the control group.

In answer to the second research question, the teachers' perceptions of the conversational program were primarily positive, as ascertained from interviews and questionnaires. They felt that they benefited from the training and that their students' vocabularies benefited from the implementation of the program.