The purpose of this study was to develop a measure for use in assessing musical behaviors of preschool children in the context of regular music instruction and to determine the validity and the reliability of the measure. The Early Childhood Musical Behavior Measure (ECMBM) was constructed for use with preschool-aged children to measure their musical behaviors during music class. ECMBM is a rating scale with 12 continuous dimensions for use in assessing the following: Tonal Skills (Singing Accuracy, Resting Tone, Major Pattern Imitation, Minor Pattern Imitation); Rhythm Skills (Chant Performance, Duple Pattern Imitation, Triple Pattern Imitation, Rhythm Improvisation, Keeping Beat: Gross Motor, Keeping Beat: Manipulatives, Keeping Beat: Locomotor); and Movement. I conducted a pilot study to practice and refine the assessment techniques and instruments that I would be using in the main study and to determine the preliminary reliabilities of the scale. Also, the measure was checked for content validity both subjectively and objectively with seven university professors who are experts in early childhood music education and music education measurement. Based on the results of the pilot study, comments from the judges in the pilot study, and comments from validation panel members, ECMBM was revised for use in the main study. To determine the reliability of the measure, the musical behaviors of 4- and 5-year-old children in two intact classes were audio and/or video recorded, and three independent judges rated the children's musical behaviors. These ratings were correlated to determine interjudge reliabilities. Also, I correlated my in-class ratings of the children's performances with my after class ratings from listening and watching to the recorded data to obtain the intrajudge reliabilities and to determine whether the ratings that I made during class were sufficiently reliable. On the basis of the results of this study, it is reasonable to state that early childhood music teachers who use a Music Learning Theory curriculum can use ECMBM to assist them in assessing preschool children's musical behaviors in the context of music instruction. The intrajudge and interjudge reliabilities indicate that the Tonal Behavior rating scales are reliable and Rhythm Behavior rating scales also are reliable except for the Chant Performance dimension. The content validation process as well as the intercorrelations provided evidence that they also are valid. The Movement dimension lacked content validity, and its ratings also resulted in low to moderate reliabilities. There is a need for further research that pertains to the refinement of the rating scales and assessment activities for music instruction at all levels. In addition, this study should be replicated with a larger sample, different group of children in terms of socio-economic status and racial make up, and with different criterion songs, chants, or patterns to investigate further the validity of ECMBM. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]