Abstract We report experimental evidence of ion–ion acoustic instability in laser-produced astrophysically relevant plasma. Irradiation of a copper foil with a short (2 ps) intense (1017 W cm−2) laser pulse generates dense hot plasma, whose expansion into the rarefied preplasma generates the electrostatic collisionless shock. A shock-reflected ion beam penetrates through the shock upstream excites the ion–ion acoustic instability. The shock oscillation electric field and the instability filamentary modulation are monitored by proton radiography. Particle-in-cell and particle-tracing simulations reproduce the experimental observation.