Using a computer-assisted molecular modeling protocol, we have completed the three-dimensional structures of HIV-1 reverse transcriptase and the Klenow fragment of DNA polymerase I based on the C alpha crystal coordinates of the individual enzymes. The two model-built structures were then used to compare the electrostatic potential contours and analyze the spatial positions of residues conserved in the catalytic domains of the two enzymes. In spite of rather weak sequence similarity and different folding patterns between the DNA-dependent DNA polymerase (pol I) and the RNA-dependent DNA polymerases (RT), we have noted the occurrence of identical or similar residues at common spatial positions in pol I and RT in a three-dimensional context. The homologous residues present at equivalent spatial position in the Klenow fragment and the p66 subunit of HIV-1 RT may therefore imply their functional similarity. Furthermore, these conserved residues may represent a similar structure-function feature in all polymerases.