Both consumptive and non-consumptive processes have been well-studied in stream ecosystems.  Predatory fish consume individuals of certain mayfly species and can cause earlier emergence and smaller body size in mayfly adults.  Previous studies of non-consumptive effects have generally focused on one or two prey taxa and few studies have been conducted over realistic spatial and temporal scales.  Here, we report preliminary results of a 16-week experiment designed to test for the occurrence of non-consumptive predatory effects on multiple prey taxa over a large spatial scale in a natural stream.  We added brook trout chemical cues to simulate predator presence in a 200-m reach of a fishless stream and measured the size at maturity of several mayfly taxa.  The size of mature individuals of Rhithrogena robusta and Baetis bicaudatus collected on day 34 of the experiment did not differ between treatment and reference reaches.  Similarly, late-emerging Epeorus deceptivus and Cinygmula spp. collected between days 57 and 73 also did not differ in body size between treatment and reference reaches.  These results indicate that not all mayfly taxa respond to the presence of a predator, and that non-consumptive predatory effects are not necessarily detectible in complex, natural ecosystems.