The present work investigates, in situ, the in vitro bioactivity of partially crystallized 45S5 Bioglass® (BG) as a function of immersion time in a simulated body fluid (SBF) using atomic force microscopy (AFM). The results obtained for the crystallized BG® were compared to those of hydroxyapatite c- and a-faces. The calcium phosphate layer grows on the crystallized 45S5 B® by multiple two-dimensional nucleation and fusion of these two-dimensional islands, which is essentially the same mode as for the hydroxyapatite c-face. The surface of the crystallized 45S5 BG® was almost fully covered with a dense and compact calcium phosphate layer after 24 h. The calcium phosphate formation on the crystallized BG® arises from a low surface energy of the surface layer and/or an effect of the layer to lower the resistance when the growth units of calcium phosphate incorporate into the growing island. These results indicate that the crystallized 45S5 BG® is suitable to be used as a filler for polymeric matrix bioactive composites, as it maintains a high bioactivity associated with a stiffer behavior (as compared to standard BG®).