The present research aimed at studying the fundamental frequency of voiced phonemes in Persian-Mazandarani bilinguals in 2007. The hypothesis was that the differences between the average of FO of voiced phonemes in bilinguals in Mazanadarani and Persian are significant. The descriptive-analytic approach was applied and no intervention was administered. The informants were 3 men and 2 women aged between 25 and 35. A test was developed by the researchers. Its content Validity was accredited by professionals in the field. The informants were asked to pronounce 57 words in Mazandarani and 57 words in Persian three times. So the voiced phonemes were studied in the beginning, final and middle positions of different words. The average of FO for each phoneme was measured using Dr. Speech software. After estimating the average of FOs for each voiced phoneme, and each informant, T-test was used to assess the significance of the differences of FOs. The difference between FOs of voiced phonemes in Persian and Mazandarani was not significant (p>0/05). However,the differences between the average of FOs between men and women were found to be significant (p<0/05 in Mazandarani and p<0/01 in Persian). The null hypothesis (Ho) of the study was proved. In other words, in Persian-Mazandarani bilinguals the differences between averages of FOs in voiced phonemes are not significant, but in male and female informants the differences proved to be significant in both languages.
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