Many tree species demonstrate altered foliar chemical composition when grown under elevated CO2 conditions, decreasing the nutritional quality of leaves for herbivores and detritivores. Leaf litter comprises a substantial portion of the organic input into some headwater stream ecosystems, so changes in the chemistry of leaf detritus may affect the food-selection behavior of organisms, such as crayfish, that feed on it. Detritus from trembling aspen (Populus tremuloides) was produced under either the current CO2 concentration of 360 ppm (ambient, AMB) or twice the current concentration, 720 ppm (elevated, ELEV). A Y-maze was used to determine crayfish preference for AMB or ELEV detritus. Experimental conditions consisted of: 1) fresh detritus, 2) detritus that had been leached in water for 24 h, and 3) leachate from detritus (dissolved organic matter, DOM) made into a slow-releasing gelatin block. Pairwise combinations of stimuli (AMB, ELEV, and a no-stimulus control, CON) were compared within each of the experimental conditions. Chemical analyses (%C and N, C:N, and % total phenolics) were done for each stimulus. Behavioral parameters measured from videotapes included initial arm choice (X2 test), proportion of time spent in each arm, and proportion of time spent at each stimulus source (arcsine transformed, paired t-tests). Percent C, C:N, and % total phenolics were significantly higher and %N significantly lower in both fresh detritus and leachate produced from leaves grown at elevated CO2. ELEV-leached detritus showed a significantly higher % total phenolics than the AMB-leached detritus. Crayfish preferred AMB over ELEV or CON when offered either fresh detritus or DOM gelatin. There were no differences in preference for ELEV vs. CON for all 3 experimental conditions. Crayfish showed no preference in any treatment when offered leached detritus. These results demonstrate that crayfish can discriminate chemically between AMB and ELEV detritus, that AMB detritus is preferred, and that crayfish are attracted by chemicals diffusing from the detritus.