The purpose of this study was to identify characteristics of typical acquisition of the Mexican Spanish stop-spirant alternation in bilingual Spanish–English speaking children and to shed light on the theoretical debate over which sound is the underlying form in the stop-spirant allophonic relationship. We predicted that bilingual children would acquire knowledge of this allophonic relationship by the time they reach age 5;0 (years;months) and would demonstrate higher accuracy on the spirants, indicating their role as the underlying phoneme. This quasi-longitudinal study examined children’s single word samples in Spanish from ages 2;4–8;2. Samples were phonetically transcribed and analyzed for accuracy, substitution errors and acoustically for intensity ratios. Bilingual children demonstrated overall higher accuracy on the voiced stops as compared to the spirants. Differences in substitution errors across ages were found and acoustic analyses corroborated perceptual findings. The clinical implication of this research is that bilingual children may be in danger of overdiagnosis of speech sound disorders because acquisition of this allophonic rule in bilinguals appears to differ from what has been found in previous studies examining monolingual Spanish speakers.