A surface viable count of Dermatophilus congolensis colonies on blood agar was found to be a satisfactory method of assaying zoospores in suspension. A filter system using Millipore filters was set up and the movement of zoospores across the filter was studied under different conditions. The movement was shown to be a true migration, rather than a random diffusion. The zoospores showed a positive chemotaxis to CO₂, with an optimum level above which migration started to decrease. The migration towards CO₂ was the same in PBS as in water. Zoospores harvested from BA plates, which had been incubated for different lengths of time, showed decreased migration as the culture time increased. The results of the experiments were compared with those of other workers who used more subjective methods of observation. The results agree with some of their findings, but more extensive experiments will have to be carried out before one can say how the zoospore behaviour is related to the pathogenesis of dermatophilosis. It was concluded that the filter system provided a quantitative method of observing zoospore behaviour and was therefore a great improvement on the methods used previously.