A mathematical model has been used to predict the effect of fibre aspect ratio on the mechanical properties of flexible polyurethane elastomers reinforced with short, microscopic fibres. The study suggested that only a small fraction of the mechanical property improvement possible has been realized in previous attempts to reinforce polyurethane elastomers using microfibres of modest aspect ratios. It was predicted that the mechanical properties of flexible polyurethane elastomers could be improved significantly by reinforcement with small amounts of microscopic fibres having high aspect ratios. Samples were prepared and characterized to test these predictions. Good agreement was seen between the theoretically predicted property trends and those actually observed. It was observed that only 2 phr§ of microscopic bacterial cellulose fibres having high aspect ratios nearly doubled the Young's modulus and tripled the strength of a flexible polyurethane elastomer. This reinforcement far surpassed that observed for microfibres having more modest aspect ratios.