An apparatus consisting of two spherical resonators of nominal internal radius (25 and 45) mm was constructed and used to determine the speed of sound in argon at temperatures between (250 and 350) K at pressures below 7 MPa. The estimated expanded uncertainty in the speed of sound determined was less than +/- 0.03 m.s(-1), that is, <+/- 8.10(-5).u, and the results optained from both resonators agreed within the combined uncertainty. Independent measurements and predictions, obtained from an equation of state (J. Phys. Chem. Ref Data 1999, 28, 779-850), of the speed of sound differed from the results by <+/- 5.10(-5).u at p < 2.5 MPa and at p approximate to 6.8 MPa by 4.10(-4).u. The second acoustic virial coefficient for argon obtained from both resonators agrees within the combined expanded uncertainty except at T = 350 K where the difference is about three times the combined expanded error.