Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties(1,2) that depend sensitively on both size and shape(3,4), and are of both fundamental and technological interest. In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate(5-12). And, in the case of optically active II-YI and III-V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects(6,8-10). Thus, except for some metal nanocrystals(13), there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions. For semiconductors, a benchmark preparation is the growth of nearly spherical II-VI and III-V nanocrystals by injection of precursor molecules into a hot surfactant(14,15). Here we demonstrate that control of the growth kinetics of the II-VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one. This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments(16,17) and as chromophores in light-emitting diodes(18,19).