The ica genes which encode proteins required for biofilm formation are considered important for the virulence of staphylococci. This study analysed genetic and phenotypic variations of biofilm formation in 70 clinical Staphylococcus capitis isolates from children. Ten were biofilm-negative as confirmed by growth in TSB with 4% NaCl and colonial appearance on Congo Red Agar. Sequence analysis of the ica operon (icaR-icaC) showed that the five genes (icaR, icaA, icaD, icaB and icaC) of S. capitis are closely related to those of S. caprae, S. epidermidis and S. aureus (≥ 70% similarity). The polypeptides predicted from the S. capitis ica genes exhibited 71% - 90% amino acid identity to those encoded by S. epidermidis, S. aureus and S. caprae ica genes. The ica operon was analysed in all 70 strains by PCR. All strains were shown to carry an ica operon of the same size and in the same relative position, suggesting that the absence of biofilm formation is not related to the insertion of a mobile element such as IS-256. HindIII or TaqI digestion of PCR products from 10 biofilm-positve ica clusters showed a uniform restriction pattern. In contrast, digestion of the ica operon of 10 biofilm-negative isolates showed 3 different patterns. Preliminary sequence analysis of one strain of each pattern revealed that a deletion of a 9-bp sequence from a highly conserved region in icaA in strain 17 and several point mutations in the icaA gene of strains 44 and 65. It is concluded that mutations may be a primary mechanism that govern the variations of biofilm formation in S. capitis.