Abstract This article contains an edition of a 17th-century Low German letter addressed to the German congregation in Stockholm. Seen against the shift of writing language from Low to High German, this letter is analyzed in respect to code-mixing, which is shown to fulfill a communicative function. Furthermore the author here suggests that the code-mixing observed in the letter can be described as congruent lexicalization, where Low German syntactic structures are filled with both Low and High German lexical material.