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T7 DNA polymerase^, catalyses DNA replication in vitro at rates of more than 100 bases per second and has a 3′→5′ exonuclease (nucleotide removing) activity at a separate active site. This enzyme possesses a ‘right hand’ shape which is common to most polymerases with fingers, palm and thumb domains^,. The rate-limiting step for replication is thought to involve a conformational change between an ‘open fingers’ state in which the active site samples nucleotides, and a ‘closed’ state in which nucleotide incorporation occurs^,. DNA polymerase must function as a molecular motor converting chemical energy into mechanical force as it moves over the template. Here we show, using a single-molecule assay based on the differential elasticity of single-stranded and double-stranded DNA, that mechanical force is generated during the rate-limiting step and that the motor can work against a maximum template …