Digital halftoning is used to reproduce a continuous tone image with a printer. One of these halftoning algorithms, error diffusion, suffers from certain artifacts. One of these artifacts is commonly denoted as worms. We propose a simple measure for detection of worm artifacts. The proposed measure is evaluated by a psychophysical experiment, where 4 images were reproduced using 5 different error diffusion algorithms. The results indicate a high correlation between the predicted worms and perceived worms.