Intelligibility is a critically important aspect for effective oral communication. This study explored salient (important) pronunciation errors and their relative roles in the intelligibility of nonnative speech based on listener judgments. Speech samples, collected from speakers with a Turkish as a native language (L1) background, were presented to listener groups in two consecutive sub-studies. In expert listening sessions, listeners with an academic background in pronunciation research and teaching (N=3) detected salient pronunciation errors via a thinking aloud protocol. During the intelligibility research, native listeners (N=33) transcribed the target words with salient pronunciation errors presented either as individual words isolated from their contexts or in longer utterances. The intelligibility scores were calculated and analyzed to investigate the role of salient pronunciation errors on speakers' intelligibility in English. The findings indicated that mispronunciations of segmentals and incorrect stress placement were the most common error types as detected by expert listeners. Negative transfer was found to be an important factor in such salient errors. According to the results of the intelligibility research, pronunciation errors affected speakers' intelligibility to a certain extent. Segmental errors were the main causes of reduced intelligibility. Moreover, mispronunciations of two sounds that were seen in most error sites, /[schwa]/ and /[theta]/, seemed to make speech more accented while not seriously affecting intelligibility. Also, errors in strong syllables had a more detrimental effect on intelligibility compared to those in weak syllables.