Abstract: We report magnetoelastic studies in the CeCo1−xFexSi alloys. Iron doping has a profound effect on the low temperature linear magnetostriction. At T = 2 K, the strength of the magnetostriction peaks at x = 0.23 where it reaches a value as large as $\[\frac{{\Delta L}}{L} = 3 \times {10^{ – 3}}\]$ in an applied magnetic field B = 16 T. This Fe concentration corresponds to the critical one xc above which long-range antiferromagnetic order is no longer observed. The progressive increment of the hibridization between the Ce 4f orbital and the conduction band, as the magnetic order vanishes, gives rise to a sizeable valence susceptibility that can be finely tuned by the magnetic field explaining the large magnetostrictive effect around xc. At higher x, the magnetostriction decreases, in agreement with a weaker valence susceptibility resulting from a stronger hibridization.