Should an alphabetic orthography for a tone language include tone marks? Opinion and practice are divided along three lines: zero marking, phonemic marking and various reduced marking schemes. This paper examines the success of phonemic tone marking for Dschang, a Grassfields Bantu language which uses tone to distinguish lexical items and some grammatical constructions. Participants with a variety of ages and educational backgrounds, and having different levels of exposure to the orthography, were tested on location in the Western Province of Cameroon. All but one had attended classes on tone marking. Participants read texts which were marked and unmarked for tone, then added tone marks to the unmarked texts. Analysis shows that tone marking degrades reading fluency and does not help to resolve tonally ambiguous words. Experienced writers attain an accuracy score of 83.5% in adding tone marks to a text, while inexperienced writers score a mere 53%, which is not much better than chance. The experiment raises serious doubts about the suitability of the phonemic method of marking tone for languages having widespread tone sandhi effects, and lends support to the notion that a writing system should have "fixed word images." A critical review of other experimental work on African tone orthography lays the groundwork for the experiment, and contributes to the establishment of a uniform experimental paradigm.