This study examined the development of the rational number concept as a ratio. Preliminary to the description of the study is an introduction discussing constructivism and equilibration. The study itself tests whether equilibration theory holds, and if so, what is the nature of its "phases" and whether these are found at each of the "periods" of development. Findings include: development of the ratio concept occurs in stages which can be both chronologically and structurally differentiated and these stages can be seen as resulting from equilibration processes which can be reorganized into two distinct "periods." A detailed description of the instrument, the orange juice test, is included. (MK)