This paper describes how Chinese-English bilinguals in Taiwan use their languages in asynchronous computer-mediated communication, specifically, via Bulletin Board System (BBS) and email. The main data includes two types: emails collected from a social network and postings collected from two BBS websites. By examining patterns of language choice and language use in these two types of data, the aim of this research is to investigate how a language contact phenomenon--code-switching (CS)--is realized via different writing systems in online environment. The data reveals that there are two levels of CS: CS between languages and CS between writing systems. In terms of CS between languages, three patterns are found: CS between Mandarin and Taiwanese, CS between Mandarin and English, CS between Mandarin, Taiwanese and English. In terms of CS between writing systems, three patterns are also identified: CS between Standard Written Chinese (SWC) and Zhuyin, CS between SWC and English alphabet, and CS between SWC, Zhuyin and English alphabet. This paper presents the two levels of CS by using the examples found in the data and describes the complex relationship between different writing systems and different languages. It is concluded that CS is found to be a common phenomenon on asynchronous CMC, where it fosters multilingualism and multi-orthography. (Contains 7 tables and 4 endnotes.)