To achieve low percolation thresholds in single wall carbon nanotube (SWCNT) and thermoplastic poly(butylene terephthalate) (PBT) composites, we have used an in situ polycondensation reaction process. The intense dispersion process achieved first by ultrasonication and followed by ultrahigh speed stirring of single wall nanotubes in 1,4-butanediol and the subsequent in situ polycondensation has made possible the preparation of nanocomposites in which the percolation threshold is around 0.2 wt % of SWCNT. This relatively low value approaches those reported for carbon nanotube nanocomposites based on thermoset polymers. On the basis of the structural measurements, we interpret that agglomeration effects may enhance the formation of the conducting network.