The validity of the theory of crucial similarity in language interference is investigated. The theory proposes that when a first and a second language are structurally similar in some aspects, the second language learner will assume similarity in other aspects, causing interference. In this study, the German of first grade students whose teacher was a Standard German speaker was compared with the German of second grade students who had previously been taught by the same first grade teacher but whose second grade teacher spoke the Swabian dialect. Error patterns in phonology and morphology, the two areas of greatest difference between Standard and Swabian German, were found to be attributable to either developmental differences or to interference due to crucial similarity. More errors were attributable to interference in phonology than in morphology. According to the theory of crucial similarity, the errors attributable to interference will also persist over time. (MSE)