Recent research has demonstrated the potential of pregnancy diagnosis in elk (Cerlvus elaphus nelsoni) using immunoassays of fecal steroid concentration. However, multiple sampies are required to insure accurate results, limiting its utility for free-ranging animals. We attempted to develop an accurate one-sample pregnancy diagnosis using 153 fecal samples that were collected from free-ranging, radio-collared, adult female elk in Yellowstone National Park (Wyoming, USA) and from captive elk maintained at the Starkey Research Facility (La Grande, Oregon, USA) February through April 1992 and 1997. The pregnancy status of each animal was diagnosed using serum pregnancy-specific protein B (PSPB) assays providing fecal samples from 38 nonpregnant and 115 pregnant animals. Fecal radioimmunoassay (RIA) indicated that mean (± SD) progestagens (P₄) were elevated significantly in pregnant (2.96 ± 1.49 μg/gm) compared to nonpregnant (0.43 ± 0.26 μ/gm) individuals. Confidence intervals (1.96 ± SE) for the two groups were widely separated (nonpregnant 0.34–0.51, pregnant 2.69–3.24) with little overlap in the range of concentrations measured for each group (nonpregnant 0.09–0.98, pregnant 0.90–8.29). These results indicate that fecal progestagens RIA provides a reliable method of noninvasive pregnancy diagnosis using single fecal samples collected from elk during late gestation. However, independent validation of the suggested discrimination criteria should be performed before routine application.