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An ion sensitive field effect transistor (ISFET) array for the direct observation of physiochemical activity in a living biological specimen of a slime mould is demonstrated. The chip comprises a 64 × 64 pixel array of 10.2 × 10.2 μm sensors. The device was fabricated in a standard 0.35 μm foundry CMOS process. Using this device it was possible to observe periodic variations in pH consistent with the biological activity of slime mould with oscillatory amplitudes of 450 μV × 70 μV peak-to-peak, equating to 0.022 pH units at 20 mV/pH sensitivity, when taking data at a rate of 400 μs per pixel-sample. These oscillations arise from intracellular Ca2+ variations giving rise to an extracellular pH change.