Two fiber/polymer composite systems provided an opportunity to determine the dominant properties that set the behavior of the systems in extensional flow. Industrial users are interested in these systems for the savings of process time and labor they may bring to high strength composite construction. To understand the basic behavior of highly aligned fiber systems, the paper presents a model system designed to provide extensional data at low and moderate fiber aspect ratio and a high aspect ratio system. The model system shows the trend to a shift from neat polymer elongational response to a shear flow dominated response as the aspect ratio increases. The high aspect ratio system demonstrates the effect of extreme filament length on the transient response.