Lower Limit to the Interstellar ^{12}C/^{13}C Ratio in the Direction of 20 Tauri

The spectrum of 20 Tau has been photoelectrically scanned in the vicinity of 4232 of interstellar Cli+. From the observed absence of the isotopically-shifted line of Cli+ it is deduced that the 12C/12C isotopic abundance ratio in this direction is greater than 33, and hence consistent with the terrestrial value.