In this work, the crystallization tendency of three bioactive glasses called bioglass 45S5, 1-98, and a new glass, 07-04 is discussed. The glasses were heat-treated in an electric laboratory furnace in nitrogen for different periods of time between 600 and 1000 degrees C, after which the samples were studied with SEM-EDXA and X-ray analysis. Glass 45S5 showed surface crystallization of sodium-calcium-silicate at 600 degrees C. At higher temperatures the glass crystallized homogenously. This suggests that also internal nucleation should be considered for 45S5. In glass 1-98 wollastonite nucleated in the surface around 800 degrees C. The thickness of the crystallized surface layer increased linearly with time and temperature of the heat treatment. The third glass 07-04 showed, depending on temperature, sodium-lime-silicate, diopside and wollastonite crystals. Sodium-calcium-silicate nucleated around 700 degrees C in the surface. With increasing temperature wollastonite and diopside formed simultaneously with sodium-calcium-silicate. At the highest temperatures the main crystalline type in the surface was diopside. The results indicate that sodium-calcium-silicate crystals nucleate and grow at lower temperatures than wollastonite or diopside crystals.