The activity of a homogeneous DNA polymerase from the thermophilic archaebacterium, Sulfolobus acidocaldarius, on a singly primed, single-stranded recombinant phage M13 DNA has been examined. At the optimal temperature (70 to 75 degrees C) this template is efficiently replicated in ten minutes using a ratio of enzyme molecule to primed-template of 0.8. Analysis of DNA products during the course of polymerization shows that species of quite homogeneous size are observed and that the number of primers extended by the enzyme is constant, whatever the enzyme molecule to primed template ratio is in the range 1/50 to 2, indicating that the 100 x 10(3) Mr DNA polymerase from S. acidocaldarius is randomly recycled on the template molecules. At non-optimal temperature (60 degrees C and 80 degrees C) the distribution of products observed indicated the presence of arrest sequences; some have been shown to be reversible. One of these pausing signals detected at 80 degrees C has been further analysed, and has been found to be DNA sequence-dependent.