The cells of the dipteran insects Chironomus and Drosophila contain high-mobility-group proteins (HMG) that are homologous to the HMG1 proteins of mammals, but contain only one HMG1 box instead of two. The C-terminal portions of both mammalian and insect HMG1 proteins comprise many charged residues that modulate the DNA-binding affinity of the HMG1 boxes and were found in Chironomus to be substrates for protein kinase C (PKC) in vitro and in vivo. Phosphorylation of Chironomus HMG1 proteins cHMG1a and cHMG1b by PKC resulted in a tenfold and fivefold reduction, respectively, of the DNA-binding strength. Phosphorylated and unphosphorylated cHMG1a protein was labelled with fluoresceine isothiocyanate and microinjected into the cytoplasm of Chironomus salivary gland cells. The translocation of phosphorylated cHMG1a into the nuclei was found to be remarkably delayed as compared to that of the unmodified form. The distribution of HMG1 proteins between nucleus and cytoplasm is known to vary according to the cell type and the state of differentiation. Our results suggest that this distribution may be regulated by changing the efficiency of nuclear translocation and the affinity for DNA via phosphorylation and dephosphorylation.