This article reports a quasi-experimental study of the effects of improvisation on the development of children's creative thinking in music. The study was conducted in a primary school classroom with two matched groups of 6-year-old children over a period of six months. The music lessons for the experimental group were enriched with a variety of improvisatory activities, while those in the control group did not include any improvisation, but instead were didactic and teacher-centred. Children in the experimental group were offered several opportunities to experience improvisation through their voices, their bodies, and musical instruments. Webster's Measure of Creative Thinking in Music-MCTM II (Webster, 1987, 1994) was administered before and after the six-month teaching programmes (i.e., pre-test and post-test) to assess children's creative thinking in terms of four musical parameters: extensiveness, flexibility, originality, and syntax. Analysis revealed that improvisation affects significantly the development of creative thinking; in particular, it promotes musical flexibility, originality, and syntax in children's music-making. (Contains 2 tables and 4 figures.)