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This study examined the quantitative variation of biofilm formation and its relationship to multilocus genotypes in 115 strains of the human pathogenic fungus Candida albicans. These strains were isolated from three sources: 47 from oral cavities of healthy volunteers, 31 from the environment and 37 from the vaginas of patients with candidiasis. For each strain, biofilm formation was quantified as the ability to adhere to and grow on polystyrene plastic surfaces. Confocal laser scanning microscopy was used to visualize and confirm biofilm formation. Twomethods were used to quantify biofilm formation abilities: (i) the XTT reduction assay, and (ii) absorbance following staining by crystal violet dye. Results obtained by the two methods were significantly correlated. Furthermore, biofilm formation ability was positively correlated with cell surface hydrophobicity. The analyses indicated that strains from each of the three sources varied widely in biofilm formation abilities. However, little correlation was observed between biofilm formation and multilocus genotypes as determined by PCR-RFLP at 16 polymorphic loci, regardless of source of strain. Strains with the same or similar multilocus genotypes often showed very different biofilm formation abilities. The results demonstrated that natural clones and clonal lineages of C. albicans exhibited extensive quantitative variation in biofilm formation