Dispersions of multiwalled carbon nanotubes (MWNT) in unsaturated polyester resin (UPR) were studied by rheology and optical microscopy. The experimentally determined percolation threshold of 0.097 vol % was very close to the theoretical value of 0.085 vol % expected for individually dispersed MWNT with an average aspect ratio of 590. Rheological analysis showed that the dispersions formed an open network with a fractal dimension of 1.28. The results were compared with single-walled carbon nanotubes (SWNT) and polystyrene (PS)-modified MWNT (MWNT-PS). The SWNT formed stronger networks than MWNT, but the MWNT-PS networks were weaker than unmodified MWNT.