Study was made of the ability of calf thymus DNA polymerases alpha and beta to replicate templates containing a small gap. It was found that during extensive replication of activated DNA or synthetic template.primers or specially prepared circular DNA containing a small gap, catalyzed by DNA polymerase alpha, the levels of incorporated nucleotides corresponded to the amounts of the single-stranded fraction of these templates. In contrast, in the reaction catalyzed by DNA polymerase beta the amounts of products were several times greater. The ability to synthesize the product in a great excess was a specific feature of the latter enzyme. An analysis of the gap-filling products by sucrose gradient centrifugation, gel electrophoresis and Southern hybridization showed that, contrary to DNA polymerase alpha, DNA polymerase beta exhibited the ability to synthesize DNA not only within but also beyond the gap. The "net" DNA product is complementary to the template strand. It is suggested that DNA was synthesized beyond the gap by displacement of the non-replicated strand.