Pressure dependence measurements of ultrasound velocities in the monocrystalline Invar alloy Fe₇₂Pt₂₈ have revealed normal behaviour in the paramagnetic phase but marked longitudinal mode softening in the ferromagnetic phase: below T_c both ( delta C₁₁/ delta P)_P=0 and ( delta B^S/ delta P)_P=0 are negative due to the magnetoelastic interaction. This is the first experimental evidence for negative longitudinal acoustic mode Gruneisen parameters in the ferromagnetic phase-as recently suggested from itinerant electron magnetism theory-and accounts for the negative thermal expansion.