There has always been dissatisfaction by language teachers who observe different levels of achievement with students attending the same language class and taking the same instruction. This variation in attaining the desired level of proficiency can be caused by the learners' personality types. The present study was an attempt to explore the degree to which one's temperament and personality type can influence one's language achievement. To this end, Longman TOEFL Test (2005) was administered to a group of 127 intermediate English learners at Marefat English Institute in Shiraz. At the same time, Black Dog Institutes' temperament and personality questionnaire was administered. After the instruction, which took about nine months, the remaining 76 students who passed three successive terms at the institute, took the same TOEFL test. The comparison between the students' achievement and their personality types revealed that there is a relationship between the learners' temperament and their language attainment. This implies that taking the same instruction, learners who are more irritable, anxious, personally reserved, and socially avoiding achieve less. On the other hand, learners who are more self-focused, interpersonally sensitive, perfectionist and self-criticizing enjoy a higher level of language proficiency.