ABSTRACT Fluorescently labelled contractile proteins (alpha-actinin and filamin) were used to study the dynamic nature of three types of microfilament bundles: myofibrils, stress fibres and polygonal networks. Cultured muscle and non-muscle cells that were microinjected with fluorescent alphaactinin rapidly incorporated the labelled protein into Z-bands, stress fibre densities and the polygonal foci. Living, injected cells were then observed for varying periods of time, and changes in orientation and periodicity of the myofibrils, stress fibres and polygonal networks were recorded. Permeabilized cells were also reacted with fluorescently labelled proteins and with contractile protein antibodies in order to analyse further the changes taking place in the myofibrils and stress fibres. In both living cardiac myocytes and living skeletal muscle myotubes, contractile myofibrils were present in the same cell with non-contractile nascent myofibrils. The periodicities of small Z-bodies in the nascent non-contractile myofibrils were shorter than the Z-band spacings in the contractile myofibrils, yet both types of myofibrils contained muscle myosin. Over a period of 24 h, a nascent myofibril in a living, microinjected myotube was observed to grow from Z-body spacings of 0·9–1·3µm to full sarcomere spacings (2·3um). During the same time, nascent myofibrils appeared de novo and Z-band alignment became more ordered in the fully formed myofibrils. Stress fibres were not observed to undergo the predictable type of growth seen in myofibrils, but stress fibre periodicities did change in some fibres; some shortened while others lengthened. The orientation of fibres shifted in cytoplasm of both mobile cells and stationary cells. Attachment plaques and foci also changed position and in some cases subdivided and/or disappeared. Models of stress fibres and polygonal networks are presented that suggest that the changes in the periodicities of the dense bodies in stress fibres and the distances between polygonal foci are related to the movement of the interdigitating actin and myosin filaments.