Foreign accent can be conceived as a deviation from a standard norm of pronuntiation. It also can be approached from the point of view of experimental phonetics. Bilingual speakers often show deviations that can be characterized at a subphonemic level by using acoustic phonetic techniques. This paper studies the vocalic system of bilingual Catalan - Castilian speakers when they use Castilian as a second language. 15 male and 15 female subjects were chosen and they recorded 33 words embedded in carrier sentences containing three occurrences of each of the five Castilian vowels in both stressed and unstressed position. Vowel formant frecuencies were derived by means of an acoustic analysis carried out with a narrow band spectral analyser Brüel & Kjaer 2033 giving a measurement error of +/- 12.5 Hz. Fl vs F2 plots were drawn using the mean values thus obtained. A comparison between vowel distribution in native monolingual Castilian speakers and in bilingual Catalan speakers using Castilian shows that: (a) [i], [a] and [u] appear to be more open for bilingual Catalan speakers; (b) in this same group of speakers [e] and [o] are closer to [i] and [u] than in native Castilians. It is concluded that bilingual Catalan subjects do not fill in the relatively large space available between the completely close and the completely open vowels in Castilian, restricting themselves to the area where native Catalan close vowels are usually found.