To study the basic mechanisms of toxicity of inhaled fibrous aerosols, fibers of specified length and diameter must be prepared. Currently, the best results in separating fibers by length are obtained following an initial aerodynamic separation by a virtual impactor. To optimize the virtual impactor performance, computer simulations are preferred because they are relatively easy and inexpensive compared to experimental trials. Prior to optimization, however, the flow fields obtained from the simulations must first be checked to show satisfactory agreement with experimental observations. The objective of this study was to obtain the flow streamlines within a water model virtual impactor by applying a flow visualization technique. Experimental results showed that the ratio of the central clean water flow to the total flow, r(c), was the most important factor affecting flow stability at the dichotomous region and should be kept at less than 5% of the total flow. The dye streams (Reynolds number, Re, > 2000) broke up in the major flow earlier than in the minor flow, indicating the development of turbulence. The turbulent flow pattern became more obvious with increasing Reynolds number. The flow at the dichotomous region was stable when the Reynolds number was greater than 2000 and less than 7000 with r(c) less than 5% of the total flow and a minor flow ratio, r(m), approximately 20% of the total flow.