Laser-induced fluorescence (LIF) peaks near 470 nm (‘blue’) and 650 nm (‘red’) from sheep feces treated with chloroform (CHCl₃) displayed spectral signatures that varied with diet. Fecal samples were obtained rectally over 3 days in a replicated study consisting of four diets (treatments) fed to 16 Polypay and Polypay × Rambouillet lambs (four lambs per treatment). Basal diet consisted of tobosa (Pleuraphis mutica Buckley [formally Hilaria mutica (Buckl.) Benth.] hay with 0% (control), 10%, 20% or 30% tarbush (Flourensia cernua D.C.) added on a dry matter basis. Feces were dried at 60 °C, treated with chloroform and the extract from both crushed and intact fecal material was then excited by laser input at 355 nm. Tarbush leaves gave LIF only in the blue region of the spectrum. However, tobosa hay and all fecal pellets produced a bimodal LIF spectral distribution with a lower, broader peak feature in the blue region and a narrower well defined taller feature in the red region of the spectrum. Chloroform did not exhibit fluorescence peaks between 400 nm and 800 nm. As tarbush in the diet increased, so did the magnitude of counts in the blue region of the spectrum. The LIF red/blue intensity ratios appear to be more reliable than actual counts in differentiating feces of sheep fed tobosa diets containing differing amounts of tarbush. red/blue ratios decreased (P< 0.05) linearly (P< 0.005) for both crushed and intact sheep fecal pellets as the tarbush/tobosa ratio in the diet increased. Inconsistencies in count amplitudes and wavelengths in either one or both regions (red or blue) of the spectrum and in red/blue ratios (especially on Day 3 for crushed fecal samples) are discussed in terms of sample preparation procedures and sampling days. Our results suggest that further testing of the LIF technique is warranted using diets consisting of several species.