The self-diffusion coefficient D-s of dye-labeled polystyrene chains having M-w = 3.90 x 10(5) g/mol and M-w/M-n < 1.10 was measured over almost 4 decades of polymer concentration in toluene solution, from 10(-4) to 0.4 g/mL, by fluorescence correlation spectroscopy. In the dilute regime up to 5 x 10(-3) g/mL, D-s remains practically constant. The transition to the semidilute regime is observed around 0.01 g/mL, and from 0.02 to 0.4 g/mL D-s decreases strongly with rising concentration. This decrease is well represented by a power law, D-s similar to c(-1.75), as predicted by scaling and reptation theory. As an alternative, a good fit over the whole concentration range can be obtained with a stretched exponential function, D-s = Do exp-(-15c(0.65)), with Do = 2 x 10(-7) cm(2)/s and c given in g/mL.