This longitudinal study examines the acquisition of subject pronouns in third language (L3) production. Participants were 90 Basque-Spanish bilingual students, learning English as a third language in a formal setting in the Basque Country. All the participants, who had received the same amount of exposure to English, were divided into three groups according to their age, and were asked to perform an oral and a written production task, after four, six and eight years of instruction in English, to examine their behaviour in relation to subject pronoun usage. The study yields different results in both production tests. In the case of the oral task, our data provide support for a positive relationship between subject pronoun usage and the number of utterances produced. Nevertheless, there does not seem to be any steady connection between subject omissions and the mean number of words used by the different groups. Concerning the written production task, age seems to be positively related to subject pronoun usage. Furthermore, there is no significant difference in the behaviour of subject pronouns in the periods under study. The fact can be associated with language complexity, which reaches higher levels longitudinally. (Contains 7 tables.)