A P-wave detector in a single chamber ventricular cardiac pacemaker, which is adapted to perform VDD pacing, senses natural atrial activity in the form of P-waves from a standard lead implanted in the ventricle of the heart. The P-wave detector is electrically coupled to a ventricular sensing lead and, although the electrode of the lead is implanted in the ventricle, circuitry within the pacemaker perceives intrinsic cardiac electrical activity arising from all parts of the heart. The P-wave detector is adapted to differentiate P-wave signals arising within the atrium from other cardiac and non-cardiac signals using template matching techniques in which an acquired signal is compared with a previously stored P-wave template.