The paper is concerned with end-gas autoignition, subsequent knock severity and magnitude of induced gas velocity. An optically accessed single cylinder Yamaha two stroke engine was modified to give complete overhead optical access to the disc-shaped combustion chamber. Flame propagation and end-gas autoignition events were recorded using high speed natural light and schlieren photography; local gas motions, prior to and induced by the knock event, were determined using an oil droplet trajectory technique. Cylinder pressure was synchronously recorded at three positions around the cylinder head; one transducer's output being simultaneously displayed on the film.End gas autoignition generally developed from multiple centres. Autoignition was usually, but not invariably, followed by knock. The severity of knock increased as the onset of autoignition occurred closer behind the top dead centre position; knock was characterized by pressure oscillations, carbon formation and high velocity post-knock gas motions. These phenomena were relatively insensitive to mass fraction unburned at the time of autoignition.