Microfluidic flow cytometry arrangements described in the literature have used a pumped flow. However, one significant shortcoming associated with such a setup is the lack of specificity. We have addressed the issue of selectivity of detection by utilising the chemotaxis property of zoospores. These spores will swim up a concentration gradient of a suitable attractant. The device design therefore has the sensing electrodes disposed in a channel within which the solution is static, and which connects a closed reservoir containing the attractant to the water to be sampled. A concentration gradient would be established within the channel and there would be an additional gradient extending from the mouth of the channel into the water to be sampled. With no flow and a clean solution in the sensing channel, the only objects moving past the electrodes should be swimming zoospores. However, the device needs to continuously sample from the environment, implying a flow through the device and hence a transition zone where zoospores being carried by the flow could detect and swim into a gradient of attractant within a static zone. The fluid velocity within this transition zone would need to be low enough that the spores could swim across the fluid streamlines. When a stream flows around a bend which has an indentation at the corner, then, depending on the angle and shape of the corner and the fluid velocity, a part of the flow moves into the indentation and creates slow-moving eddies within which objects from the flow may be trapped. A sophisticated version of this idea has been demonstrated by Tsai et al. We have used a simple version for proof-of-concept (Figure ). The sampling efficiency of this device is not expected to be high, being determined by the fraction of the total fluid mass flow through the inlet that moves into the vortices within the stub. A device such as that described by Tsai et al, using sharp angle and a sheath flow to focus the inlet flow into the stub, would be expected to achieve a higher sampling efficiency.