Objective To observe morphological characteristics and activity distribution of T. asahii biofilm. Methods The morphological characteristics of T. asahii biofilm were observed under an inverted microscope and scanning electron microscope, and activity was measured and quantitatively analyzed by 2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino) carbonyl]-2H-tetrazo-lium hydroxide (XTT) assay and viable count, respectively. Spatial distribution of dead/vital cells, activity and thickness of biofilm at different layers were assessed under a confocal laser scanning microscope (CLSM) following double staining with FDA/PI. Results T. asahii formed a biofilm in vitro on the surface of polystyrene materials. Under a scanning microscope, the biofilm displayed a complex three-dimensional structure which composed of spores, pseudohy-pha and true hypha. As time prolonged, the activity and quantity of biofilm increased. The results of XTT assay were correlated with those of viable count (r = 0.94, P < 0.01). The activity was of no obvious difference between different layers of the biofilm. The thickness of biofilm varied from 14.3 μm to 31 μm. Conclusions The structure of T. asahii biofilm in vitro is more complex than that of planktonic T. asahii. The activity is of no significant difference between different layers of T. asahii biofilm.