This study is an auditory and acoustic investigation of the speech production patterns developed by English-Arabic bilingual children. The subjects are three Lebanese children aged five, seven and ten, all born and raised in Yorkshire, England. Monolingual friends of the same age were chosen as controls, and the parents of all bilingual and monolingual children were also taped to obtain a detailed assessment of the sound patterns available in the subjects' environment. The study addresses the question of interaction between the bilingual's phonological systems by calling for a refinement of the notion of a `phonological system' using insights from recent phonetic and sociolinguistic work on variability in speech (e. g. Docherty, Foulkes, Tillotson, & Watt, 2002; Docherty & Foulkes, 2000; Local, 1983; Pisoni, 1997; Roberts, 1997; Scobbie, 2002). The variables under study include /1/, In, and VOT production. These were chosen due to the existence of different patterns in their production in English and Arabic that vary according to contextual and dialectal factors. Data were collected using a variety of picture-naming, story-telling, and free-play activities for the children, and reading lists, story-telling, and interviews for the adults. To control for language mode (Grosjean, 1998), the bilinguals were recorded in different language sessions with different interviewers. Results for the monolingual children and adults in this study underline the importance of including controls in any study of bilingual speech development for a better interpretation of the bilinguals' patterns. Input from the adults proved highly variable and at times conflicted with published patterns normally found in the literature for the variables under study. Results for the bilinguals show that they have developed separate sociolinguistically-appropriate production patterns for each of their languages that are on the whole similar to those of monolinguals but that also reflect the bilinguals' rich socio-phonetic repertoire. The interaction between the bilinguals' languages is mainly restricted to the bilingual mode and is a sign of their developing sociolinguistic competence.