A new amperometric plant tissue biosensor based on the co-immobilisation of ground spinach leaves and ferrocene in a carbon paste matrix is described for the determination of glycolic acid. The spinach tissue acts as a source of glycolate oxidase and peroxidase. Ferrocene is employed as a mediator which facilitates electron transfer between the electrode surface and hydrogen peroxide. The glycolic acid assay is based on the amperometric measurement of a reduction current corresponding to the reduction at the electrode surface of ferricinium ion generated from the enzymatic reaction at 0.00 mV vs. Ag/AgCl. The effect of various experimental parameters such as pH, working buffer, applied potential and paste composition is explored for optimum analytical performance. The bioelectrode exhibits linear response up to 5 X 10v4 M glycolic acid with a relative standard deviation of 1.50% (n = 30) and a detection limit of 1 X lOA M. The bioelectrode responds rapidly to glycolic acid, with steady state current responses achieved in less than 1 min. The sensitivity of the bioelectrode decreased to 50% of the original value after 12 days of continuous use.