The crystallization behaviour of a ZrO2-containing glass and a ZrO2-free glass in the Y2O3-Al2O3-SiO2 system prepared at 1700°C were compared in order to study the influence of ZrO2 on the nucleation and growth processes. Techniques used included SEM, DTA and XRD analysis. The microstructural development of the ZrO2-containing material during crystallization was more complex than that of the ZrO2-free material. In the crystallization treatments no phase separation prior to crystallization could be observed and no precursor crystalline phase was observed to form in the ZrO2-containing glass. Thus the added zirconia could be considered to act as a growth modifier rather than a catalyst nucleating agent.