A very strong magnetoelastic effect in the CeCo1−xFexSi alloys is reported. The strength of the magnetostrictive effect can be tuned upon changing x. The moderate low-temperature linear magnetostriction observed at low Fe concentrations becomes very large ( ∆L L (16T, 2K) = 3×10 −3 ) around the critical concentration (xc ≈ 0.23) at which the long-range antiferromagnetic order vanishes. Upon increasing doping through the non-magnetic region (x > xc), the magnetostriction strength gradually weakens again. Remarkably the low-temperature magnetostriction at the critical concentration shows a pronounced S-like shape (centered at Bm ∼ 6 T) resembling other well-known Ce-based metamagnetic systems like CeRu2Si2 and CeTiGe. Unlike what is observed in these compounds, however, the field dependence of the magnetization shows only a minor upturn around Bm vaguely resembling a metamagnetic behavior. The subtle interplay between magnetic order and the Kondo screening seems to originate an enhanced valence susceptibility slightly changing the Ce ions valence, ultimately triggering the large magnetostriction observed around the critical concentration.