Abstract: The invention is directed to a rapid, efficient, inexpensive screening and diagnostic procedure for detecting avermectin/milbemycin resistance in parasitic nematodes. The procedure is designed for field use. The method entails the incubation of nematode sensillum with specific visible or fluorescent dyes that intercalate and diffuse in membranes of sensory neurons on the outer surface of wild-type nematodes but which do not bind or have reduced binding in resistant nematodes. The method is particularly directed to ivermectin resistant parasitic nematodes.