Believed dialect influences speech perception by linguistically naïve speakers. How much accent-induced bias affects perception of linguistically trained speakers is still unclear. This study experimentally investigates the influence of believed dialect on plosive perception by subjects who were phonetically and phonologically trained. Identical syllables were presented twice to each subject. In one session, the subjects were informed that the variety was a Mandarin dialect which has voiceless unaspirated and aspirated voiceless stops; and in the other session that it was a Wu dialect, which has voiceless unaspirated, voiceless aspirated, and breathy stops. More breathy stops were reported if Wu was the believed dialect. Plosive phonation in Wu is related to lexical tone, and the study shows that lexical tone causes another bias to plosive perception. This suggests that linguistically trained transcribers are susceptible to higher order linguistic knowledge and it demonstrates the difficulty of avoiding biased perception when the coder forms a belief about the variety that he/she transcribes. The stuy also advocates speech perception models which include a component that accounts for the role of expected sounds.