This exploratory study analyzes the written language preferences and codeswitching behavior in the diary of a Low German-High German-English trilingual speaker, which was written 50 years after his migration from Germany to the USA. Despite shifts in the spoken domain, the first language of literacy (High German) is still the primary written language, supporting Fishman's (1965: 78) observation that language maintenance may be stronger in the written domain. Interestingly, the use of Low German is expanded to the written domain, and is favored for emotional topics (e.g., childhood and aging) and self-composed poems, while English is often used for political opinions and work life. Throughout the diary, code-switching frequently occurs between paragraphs, but an increase of intra- and intersentential switches can be observed as well. These findings highlight the potential differences between spoken and written language use even within individuals. Moreover, this paper calls for a domain- or topic-specific analysis of written code-switching.