A method for tracking movement of nematode worms comprises: (i) providing a plurality of worms on a translucent substrate; (ii) obtaining a first image of a field of view including the plurality of worms by transmission imaging; (iii) obtaining a first difference image of the plurality of worms corresponding to an intensity difference between said first image and a background image of the field of view; (iv) repeating the following steps (a) to (d) a plurality of N times, for n=1 to N: (a) determining, from the first difference image, an nth pixel corresponding to a maximum intensity difference; (b) selecting, from the first difference image, an nth block of pixels comprising the selected nth pixel; (c) determining a coordinate associated with the selected nth block of pixels; and (d) updating said first difference image by setting each pixel of said nth block of pixels in said first difference image to a value corresponding to a zero or low intensity difference; (v) obtaining a sequence of M subsequent images of the field of view by transmission imaging; and (vi) repeating the following steps (f) and (g) for each of the M subsequent images, for m=2 to m=M+1: (f) obtaining an mth difference image of the plurality of worms corresponding to an intensity difference between the mth subsequent image and said background image; and (g) repeating the following steps a plurality of N times, for n=1 to n=N: determining, from the mth difference image, an nth pixel corresponding to a maximum intensity difference or a centre of the intensity difference distribution of a trial block of pixels positioned at the determined coordinate associated with the corresponding nth selected block of pixels of the (m−1)th difference image; selecting an nth block of pixels of said mth difference image, said nth block of pixels comprising the determined nth pixel; and determining a coordinate associated with the selected nth block of pixels of said mth difference image.