OBSERVATIONS of the abundances of elements provide insight into their production and distribution. The production of light elements (in particular, lithium, beryllium and boron) is dominated by spallation reactions(1), in which cosmic rays break apart more massive nuclei. Models(2,3) suggest that the B-11/B-10 ratio should be about 2.5, but the observed ratio in the Solar System is about 4 (refs 4.5). This has led to the suggestion(5) that the pre-solar nebula was subjected to bombardment by low-energy Galactic cosmic rays, leading to an overproduction of B-11 (ref. 6). Until now, it has not been possible to measure the B-11/B-10 ratio in the interstellar medium, because the lines are very weak. Here we present a spectroscopic measurement of the B-11/B-10 ratio in the interstellar gas lying between the Earth and the star delta Scorpii, made using the Hubble Space Telescope. Our measurement ratio, 3.4(-0.6)((+13)), is comparable to the meteoritic value(5), but somewhat lower. Further measurements will be needed to establish whether the ratio reported here is characteristic of the interstellar medium in general.