An approximate analytic expression for the electrophoretic mobility of a spherical colloidal particle in a symmetrical electrolyte solution is obtained, This mobility expression, which is correct to the order of the third power of the zeta -potential of the particle, considerably improves Henry's mobility formula (Proc, R, Sec. London Ser. A 133, 106(1931)) and has a much simpler form than similar mobility formulas expressed in powers of zeta derived by Overbeek (Kolloid-Beihefte 54, 287 (1943)) and by Booth (Proc, R, Sec. London Ser. A 203, 514 (1950)), Comparison with the exact computer calculation by O'Brien and White (J, Chem, Sec. Faraday Trans. 2 74, 1607 (1978)) shows that the obtained mobility formula is an excellent approximation, especially for small kappaa (where kappa is the Debye-Huckel parameter and a is the particle radius) in which region no other simple analytic mobility formulas are available than Henry's formula, which is correct to order zeta and is applicable only fair low zeta.