This paper examines the implications of different monetary and fiscal policy rules in an economy characterized by Harrodian instability. We show that (i) a monetary rule along Taylor lines can be stabilizing for low debt ratios but becomes de-stabilizing if the debt ratio exceeds a certain threshold, (ii) a 'Keynesian' fiscal policy rule can stabilize the economy at full employment, (iii) a fiscal 'austerity' rule that links fiscal parameters to deviations from a target debt ratio fails to adjust the 'warranted' to the 'natural' growth rate and destabilizes the warranted path, (iv) instability may arise from a combination of fiscal and monetary policy rules which separately would stabilize the system, and (v) austerity rules can in some circumstances enhance the stabilizing effects of monetary policy.JEL classification: E12, E52, E62, E63