A new knowledge representation scheme is proposed for representing incomplete information in relational data bases. The knowledge representation scheme introduces a novel convention for negative information based on modal logic and a novel data structure obtained by introducing tuple flags in the relational model of data. Standard and minimal forms are defined for relations conforming to the new data structure. The conventional relational operators, select, project and join, the redefined so they can be used to manipulate relations containing incomplete information. Conditions are presented for the lossless decomposition of relations containing incomplete information. 20 references.