DDP1 is a single-stranded nucleic acid binding protein of Drosophila melanogaster that associates with pericentric heterochromatin. DDP1 contains 15 consecutive KH domains and is homologous to the highly conserved vigilin proteins that, in Saccharomyces cerevisiae, are involved in the control of cell ploidy. DDP1 was identified and purified on the basis of its binding to the pyrimidine-rich C strand of the centromeric Drosophila dodeca-satellite. Here, the interaction of DDP1 with the dodeca-satellite C strand was analyzed in detail. This interaction is sequence specific. In particular, a guanine residue which is highly conserved in natural dodeca-satellite sequences was found to be essential for the efficient binding of DDP1. DDP1 binding was also found to be strongly influenced by the length and extent of secondary structure of the DNA substrate. Efficient DDP1 binding required a minimal length of about 75 to 100 nucleotides and was facilitated by the lack of secondary structure of the substrate. DDP1 also showed a significant affinity for the unstructured pyrimidine-rich strand of the most abundant centromeric Drosophila AAGAG satellite. The stoichiometry of the complexes formed with the dodeca-satellite C strand suggests that, in DDP1, the 15 consecutive KH domains are organized such that they define two nucleic acid binding surfaces. These results are discussed in the context of the possible contribution of DDP1 to heterochromatin organization and function.