Most theoretical models on the development of solidification microstructures an based on diffusive growth conditions, whereas convection effects are generally present in most directional solidification experiments. This paper examines the effect of convection on the development of microstructures in directionally solidified Al-Cu alloys. A numerical model of convection is first discussed to characterize fluid flow as a function of sample diameter for the thermal profile of the directional solidification used in this study, and it was established that sample diameters of 1 mm or smaller is required for diffusive growth. Based on this result, a new experimental design is developed for directionally solidifying several thin samples of different diameters in the same experimental run to systematically study the effect of sample diameter on microstructure development. Sample diameters, ranging from 0.2 to 5.0 mm, were used to access both the diffusive and convective regimes. Experiments have been performed over a range of velocities and temperature gradients to characterize the effect of convection on planar, cellular and dendritic microstructures. In very thin samples of less than or equal to1.0 mm diameter, diffusive growth was obtained. The cell/dendrite tip composition and primary spacing were found to increase, and the tip temperature was found to decrease, as the convection effects were reduced in thinner samples. The planar to cellular transition was suppressed in the presence of fluid Row. Also, convection was found to slightly increase the critical velocity fur cellular to dendritic transition.