Moniliformis moniliformis-infected Periplaneta australasiae and Blatta orientalis were less active than uninfected controls in behavioral arena tests under both red and white light regimes. There was a strong interaction between light regime and parasitism for substrate use by P. australasiae. Under red light, parasitism decreased the use of black horizontal surfaces, whereas under white light, parasitism increased the use of black horizontal surfaces. In contrast, B. orientalis substrate use was unaffected by parasitism, although infected animals were less active than uninfected conspecifics under both light regimes. For both species, parasitism did not change the percentage of time spent in light, the freeze time, or the directional response to light.