Error Analysis is still a very productive area in the second language acquisition field. This paper focuses on a variety of transfer errors at the levels of phonetics and syntax in the interlanguage of Basque-Spanish bilinguals learning L3 English in a formal context. We have collected data from 10 fourteen year-old Basque/Spanish bilinguals who have been learning English for 7 years in a formal school context in the Basque Country. Participants were recorded while telling a story in English and their oral productions were analysed so as to discover phonetic and syntactic transfer errors. Four different phenomena were examined at the level of phonetics – (i) replacement of novel phonemes by L1 sounds, (ii) lack of aspiration in stop sounds, (iii) spirantisation of stop sounds, and (iv) closure of fricative sounds. At the level of syntax three different features were covered – (i) use of null subjects, (ii) existence of null objects, and (iii) production of null determiners. Analyses showed that learners display L1 effects in the acquisition of both L3 English phonetics and syntax, though phonetic transfer errors were more frequent than syntactic ones (69.26% vs. 16.67%). This finding seems to indicate that 7 years of exposure to English are not sufficient to eradicate the influence of the L1/s over the phonetic and syntactic interlanguage systems. A special emphasis is placed on the pedagogical implications of the results. It is suggested that negative L1 transfer would be minimised if certain educational measures were adopted, such as (1) the enhancement of both teachers’ and learners’ linguisitic awareness towards some of the interlanguage processes examined in the paper, (2) the inclusion and in-depth analysis of contrastive linguistics and language acquisition topics in teacher training programmes, and (3) learners’ participation in courses where the target language is used in a more natural, communicative way.