Fragments produced in the reactions of 69 MeV/nucleon ³⁶Ar and 55 MeV/nucleon ⁴⁰Ar on a Be target have been measured experimentally. The isotopic distributions from fragmentation reaction for both projectiles are reproduced by using a modified statistical abrasion-ablation model. A strong isospin effect is exhibited in the isotopic distributions produced in ³⁶Ar and ⁴⁰Ar fragmentation. Experimental evidence is seen for the disappearance of the isospin effect in the fragmentation reaction.