Selenium-75 (Fig. 1) is much used as a radioactive tracer in nuclear medicine. Several thousand administered doses of selenium-75 compounds are given per year (1) in the U.S. for liver scanning and other purposes. Although accuracies of only ± 10% in activity measurements are required by the U.S. Pharmacopoeia, more accurately claibrated sources of selenium-75 are useful in nuclear medicine for the calibration of detectors in the energy region from 97 keV to 401 keV since γ-ray abundances are known to a few percent in most cases. For this and other reasons, the National Bureau of Standards (NBS) has developed, by means of an X-γ coincidence counting method, a radioactivity standard of selenium-75, and, at the same time, determined the poorly-known fraction, ƒ g , of selenium-75 disintegrations going directly to the ground state of arsenic-75. ƒ g has been calculated in this work from the difference between the measured value for the activity of a sample and its measured total γ-ray emission rate, using the NBS 8-in. NaI(Tl) detector system. (2) Most selenium-75 disintegrations produce a response in this system because of its high γ-ray detection efficiency and because most disintegrations produce two or more γ-rays. Values have been reported (3–5) for ƒ g ranging from < 10 −3 to 0.04. The Nuclear Data Tables, (6) taking a weighted average of published values, quotes ƒ g = 0.023 ± 0.023 (uncertainty of 2σ). The value of ƒ g determined by us was zero, with a 99% probability of being less than 1.2%. The measured value for the activity is independent of all decay-scheme parameters, except conversion-electron intensities (which are known to ± 4%) (6) and the probability of K capture, which appears in a small correction term.