The study examines the intelligibility of a German speaker's replacement of the initial interdental /θ/ with the alveolar fricative [s] in words that occurred in her reading of a short English text. Twenty nine students in university English courses in Sweden listened to, and transcribed the whole reading, where substituting the initial /θ/ of a word with [s]appeared four times. The result shows that the phoneme substitution by the German speaker did not cause misunderstanding in three instances, but it considerably misled the listeners' understanding of a phrase in one occasion. We discuss this finding in relation to the functional load of the initial θ/s contrast (Catford, 1987), and Jenkins' (2002, 2015) Lingua Franca Core syllabus.