This study examines the effect of native "vs." non-native prosody instruction on developing interpreter trainees' speech comprehensibility in English as a foreign language (EFL) using a pretest-posttest-delayed posttest design. Twenty-three groups of 28 interpreter trainees at a University in Iran (six different branches) took part in the study, all groups receiving the same amount of instruction (9 hours over 3 weeks). Three control groups listened to/viewed authentic audio recordings and movies in English, discussed their contents, and completed a variety of speaking tasks but received no specific prosody instruction. Twenty experimental groups spent part of the instruction time on theoretical explanation of, and practical exercises with, English prosody by thirteen nonnative instructors, and seven native instructors. Three experts evaluated the comprehensibility of the trainees in elicited speech samples collected during the pretest, immediate posttest and delayed posttest, and subsequently presented in random order. The findings revealed that the experimental groups gained between 1 and 2 points on the 0 to 10 comprehensibility scale, and lost little in the delayed posttest; however, hardly any changes were observed in the control groups. We conclude that native and non-native English instructors' prosody teaching were equally effective in enhancing EFL students' speech comprehensibility.