One of the greatest problems associated with membrane processes is the loss of productivity due to biofouling. Currently, biofouling is detected by monitoring progressive flux losses, which is an after-the-fact practice with respect to this phenomenon. One of the greatest challenges for water treatment membrane operations is the development of non-invasive methods that allow for early biofilm detection prior to irreversible macroscale flux losses, which could extend membrane life and may help reduce operation and maintenance costs. This paper presents a novel, non-invasive technique using ultrasonic frequency-domain reflectometry (UFDR), to detect biofilms on polyamide membrane coupons and polycarbonate slides. This is a variation of a technique previously reported to detect mineral scaling on RO membranes, which holds promise for real-time biofilm detection in full-scale membranes. Includes 10 references, figures.