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SUMMARY With increasing resistance to anti-parasitic drugs, it has become more important to detect and recognize phenotypes of resistant isolates. Molecular methods of detecting resistant isolates are limited at present. Here, we introduce a microfluidic bioassay to measure phenotype using parameters of nematode locomotion. We illustrate the technique on larvae of an animal parasite Oesophagostomum dentatum. Parameters of sinusoidal motion such as propagation velocity, wavelength, wave amplitude, and oscillation frequency depended on the levamisole-sensitivity of the isolate of parasitic nematode. The levamisole-sensitive isolate (SENS) had a mean wave amplitude of 135 μ m, which was larger than 123 μ m of the levamisole-resistant isolate (LEVR). SENS had a mean wavelength of 373 μ m, which was less than 393 μ m of LEVR. The mean propagation velocity of SENS, 149 μ m s −1 , was similar to LEVR, 143 μ m s −1 . The propagation velocity of the isolates was inhibited by levamisole in a concentration-dependent manner above 0·5 μ m . The EC 50 for SENS was 3 μ m and the EC 50 for LEVR was 10 μ m . This microfluidic technology advances present-day nematode migration assays and provides a better quantification and increased drug sensitivity. It is anticipated that the bioassay will facilitate study of resistance to other anthelmintic drugs that affect locomotion.